Because Levin appears first.
Levin/Lyovin is the main character of the novel: it is not just about Anna and Vronsky! See LitCharts, for example:
Levin, the other main protagonist of the novel (besides Anna)…
Screenwriting guru Michael Hauge in his article on romcom structure pointed out (it seems it’s not on the web anymore, Russian translation is here):
The protagonist must appear on the screen before his opponent. Script
readers and the audience instinctively identify themselves with the
first character to appear. If Pierce Brosnan had appeared in Mrs.
Doubtfire earlier than Robin Williams […] then we would have perceived
these movies very differently.
(So it’d have been something like “Responsible office worker must defend his love from a vengeful cross-dresser from the past!” - I don’t know?)
The same mechanism seems applicable here. How is Vronsky introduced? Earlier, we meet Levin; we see how he interacts with his pals, with Kitty, with his brother; we find out that he’s in love. We get connected to his experience. Then Stiva says (Part 1, Ch. 11)
“There’s one other thing I ought to tell you. Do you know Vronsky?” [...]
“No, I don’t. Why do you ask?” […]
“Why you ought to know Vronsky is that he’s one of your rivals.”
“Vronsky is […] Fearfully rich, handsome, great connections, an
aide-de-camp, and with all that a very nice, good-natured fellow. But
he’s more than simply a good-natured fellow, as I’ve found out
here—he’s a cultivated man, too, and very intelligent; he’s a man
who’ll make his mark.”
Levin scowled and was dumb.
So Vronsky is his rival, and Levin is not happy. And we neither – even before we learn that Vronsky is not honest with Kitty. Why good aspects of Vronsky do not count, though?
Psychologist Jordan Peterson in his 2017 lecture “Story and Metastory” shows a video from a famous Invisible Gorilla experiment. Subjects who’re trying to count the passes of the ball often didn’t notice something supposedly prominent – a person in a gorilla costume. Professor explains that to pay attention to everything around is too overwhelming for us. So our value structures and our goals limit the possible information creating a certain frame of reference. We make most of the things that don’t correspond to our goals irrelevant. We see everything else as either an obstacle or something facilitating our success.
And stories work similarly.
That is a model, obviously something can be built upon it; and when Levin meets Vronsky in person, he tries to find good in him – but the initial emotions are negative.
We identify with our protagonist and his goals, and our emotions track the progress towards the goal. And when something that complicates that progress arises – we don’t like it! So there are good and beneficial aspects of Vronsky’s personality, but they are no more visible than the gorilla in the famous test.
So readers immediately dislike Vronsky.