Why does only one of R. S. Thomas' poems conform to traditional poetic devices?

Question

R.S. Thomas was a Welsh poet who worked between 1942 until his death in 2000. His poems are almost exclusively modern in structure, in the sense that they eschew meter and rhyme in favour of a more prose-like structure. Here's a short example:

Looking upon this tree with its quaint pretension
Of holding the earth, a leveret, in its claws,
Or marking the texture of its living bark,
A grey sea wrinkled by the winds of years,
I understand whence this man's body comes,
In veins and fibres, the bare boughs of bone,
The trellised thicket, where the heart, that robin,
Greets with a song the seasons of the blood.

But where in meadow or mountain shall I match
The individual accent of the speech
That is the ear's familiar? To what sun attribute
The honeyed warmness of his smile?
To which of the deciduous brood is germane
The angel peeping from the latticed eye?

One of his poems, however, is quite different in structure:

One night of tempest I arose and went
Along the Menai shore on dreaming bent;
The wind was strong, and savage swung the tide,
And the waves blustered on Caernarvon side.

But in the morrow, when I passed that way,
On Menai’s shore the hush of heaven lay;
The wind was gentle and the sea a flower,
And the sun slumbered on Caernavon tower.

Why does this - and, as far as I can tell only this - poem, entitled Night and Morning, among his work conform to traditional poetic structures?

Answer 1

The reason is because it is not entirely Thomas' own work: it is a translation of a traditional Welsh song (obtained here).

Ar noswaith ddrycinog mi euthum i rodio
Ar lannau y Fenai gan ddistaw fyfyrio;
Y gwynt oedd yn uchel, a gwyllt oedd y wendon,
A’r môr oedd yn lluchio dros waliau Caernarfon.

Ond trannoeth y bore mi euthum i rodio
Hyd lannau y Fenai, tawelwch oedd yno;
Y gwynt oedd yn ddistaw, a’r môr oedd yn dirion,
A’r haul oedd yn twrynnu ar waliau Caernarfon.

I am not a Welsh speaker, but a cursory glance at the original suggests it is composed in rhyming couplets, as Thomas' translation is.

A more literal - non-rhyming - translation of the words makes it clear why Thomas felt entitled to publish his version as poetry of his own devising. This is from William John Gruffydd, sometimes better known as "Elerydd", the Archdruid of the National Eisteddfod of Wales.

One rainswept eventide I went a-walking
On the shores of Menai in silent meditation:
Loud was the wind and wild was the white billow,
And the sea was hurling over the walls of Caernarvon.

But on the morrow morn I went a-walking
On the shores of Menai, and stillness was on them;
Silent was the wind, and kindly was the sea,
And the sun was shining on the walls of Caernavon.

Apparently John Betjeman was moved to describe Thomas' version as "the perfect lyric".