Because the statement "A key theme of the novel is Bloom's relative social isolation" is false.
Answering this in the negative, when no evidence to the positive has been presented whatsoever, is bound to be difficult. Rationally, I should not proceed until such evidence has been provided. But here goes.
Short Argument - "Reductio ad Absurdum via Occam's Razor"
Consider Stephen. We know from the Artist that Stephen certainly feels socially isolated. Now, when we meet him again in his tower, we notice, this time with the tool of internal monologue, that, not surprisingly, Stephen thinks mostly about Stephen and his relationships to others.
Stephen bent forward and peered at the mirror held out to him, cleft by a crooked crack. Hair on end. As he and others see me. Who chose this face for me? This dogsbody to rid of vermin. It asks me too.
- Stephen's social isolation swamps his internal monologues for the rest of the novel.
Bloom is certainly personally lonely, generally since he has no son, and today, because Molly is having sex with Blazes Boylan, and this is certainly reflected in his internal monologues. For example, from Sirens,
Last rose Castile of summer left bloom I feel so sad alone.
But this is personal loneliness, and today, given Molly and Blazes, personal isolation.
On the other hand, as the OP noted, Bloom's internal monologues exhibit little, if any, sign of social isolation, the anti-Semitism he encounters notwithstanding.
This would contradict the assumption that Bloom feels socially isolated, unless there exists some extra special reason for Bloom to block it out, this despite being unable to block out his personal loneliness and personal isolation.
The OP, of course, is looking for such a special reason, but is only doing so because the OP believes a false assumption. There is no such special reason evidenced in the novel, and dropping the false assumption, we need not seek one.
Longer Arguments
Bloom, unlike Stephen, is not socially isolated, at least not beyond the kinds of isolation most of us encounter to varying degrees.
- Note that statements like the above, asserting the normalcy of Bloom are essentially tautologies, because Bloom is "Everyman" (see for example, SparkNotes). Expect no extremes from Bloom.
On the whole, Bloom chats with those he meets along the way, travels with old friends to a funeral, seeks out social settings, participates in conversations,
stands up for what he believes in (getting a biscuit tin thrown at him once well out of range), etc., and does so in a relatively normal manner.
I add relatively, because we must not forget what is really going on: Bloom is wandering the streets to avoid going home while Molly has sex with Blazes Boylan! He is in shock, numb, a controlled state of crisis. Trying not to think about it, but its always sneaking in. This is the primary force distorting Bloom's normal internal monologues, which, in contrast to Stephen's, are normally outward focussed. Bloom is numb, personally lonely, personally isolated (from Molly), but not social isolated. That is not Bloom.
Sure he is Jewish (whatever that means), but you wouldn’t notice in a public Irish urinal, synagogue, nor butchery.
Mr Leopold Bloom ate with relish the inner organs of beasts and fowls.
Counting misogyny, most people are socially isolated at Bloom's level: Bloom is every (wo)man. This level of daily racism/misogyny/homophobia/fundamentalism/xenophobia/small-mindedness/ugly-gossip, is par for the course for most, both then and now. It has not come to define him.
Bloom deals with many of these horrible/awkward moments just as we tend to.
In the tram, he "avoids" the anti-Semitism by colluding.
Mr Bloom began to speak with sudden eagerness to his companions’ faces.
—That’s an awfully good one that’s going the rounds about Reuben J and the son.
In Cyclops Bloom is pitted against the Citizen, and takes a stand for what he believes in. While this does eventually descend into anti-Semitism and a thrown tin, the episode is rather like pitting Mr Bean against a Brixiteer.
Begob I saw there was trouble coming. And Bloom explaining he meant on
account of it being cruel for the wife having to go round after the
old stuttering fool.
Note that Bloom is in fact initially welcomed by the Citizen.
Come in, come on, he won’t eat you, says the citizen.
Joe even buys Bloom a cigar. Not particularly socially isolating. Further, it is Bloom/Mr Bean who starts arguing with the citizen in the first place.
That can be explained by science, says Bloom. It’s only a natural
phenomenon, don’t you see, because on account of the...
And then he starts with his jawbreakers about phenomenon and science
and this phenomenon and the other phenomenon.
The encounter is won by the ever civic minded Bloom/Odysseus, who blinds the Citizen/Cyclops and escapes, the hurled biscuit tin not even coming close.
But, going back to Homer, the key point of the episode is that Bloom/Odysseus loses his self-control for a moment and yells. And why does he lose his composure? Not because of some feeling of social isolation, but, of course, Molly and Blazes.
—Well, that’s a point, says Bloom, for the wife’s admirers. —Whose
admirers? says Joe. —The wife’s advisers, I mean, says Bloom.
and
—Who? Blazes? says Joe. And says Bloom: —What I meant about tennis,
for example, is the agility and training the eye. —Ay, Blazes, says
Alf.
And Bloom cuts in again about lawn tennis and the circulation of the
blood,
As I have noted, no evidence has been presented that anyone of authority believes that a key theme of the novel is Bloom's relative social isolation.
I thought to well-read Hugh Kenner's chapter "The Hidden Hero", which is an overview of Bloom and his day. Nowhere do I encounter any discussion on Bloom's social isolation.
The following quotes (from that chapter) give a feel.
He is quietly witty, too, and when not preoccupied as on Bloomsday by
intolerable worry he can be what Dublin much prizes, a man of tongue.
Stature, relative wealth, an exalted dwelling-place, handsome
features, polysemous wit, a famously beautiful wife: Bloom may be
said, albeit misleadingly, to possess these salient attributes of his
prototype the Homeric chieftain.
Mulligan, we have noticed, is all
outside; the book does not grant him a hint of inner life. Stephen by
the end of 'Proteus' has become virtually all inside, the great bright
world subsumed into his phrasemaking. Bloom at first is a balance; we
move in and out, in and out, the 'out', however, closely in touch with
the 'in', prompting and controlling.
But, until 4 p.m., benumbment. For eight hours Bloom goes through engrossing
motions, busily curious, blocking off from his thoughts what ought to be the
novel's principal topic, the Boylan-Molly liaison
The man is virtually in shock.
