Does anyone know what may have possessed Moulinsart?
The short answer is that they don't "own" the Michael Turner and Leslie Lonsdale-Cooper translations - the copyright in those currently (as far as we know) reside with Egmont, who in turn inherited them when they took over the titles from Methuen.
Egmont in turn didn't have the rights to produce digital editions of the books, and Moulinsart did.
Another aspect is that while, like you, I think that the "traditional" translations are superb - having been made not just by translators with an amazing affinity for the spirit of the work, but in consultation with Hergé himself, making them - to my way of thinking - the "authorized" version, there are those who have complained that the English text does not show enough fidelity to the original French, and that a closer translation (more "word-for-word") was needed, which is what Michael Farr was tasked with doing.
I am sure it was done with the best of intentions, and the MT&LL-C texts remain available in the hard copies so we should probably think of them as complementary, rather than mutually exclusive.
I remain to be convinced however that the new texts do the albums justice, and not through actual deficiency in the translation. The appearance of words on a page - how the artist balances white space against the rest of the frame to allow text to be used effectively, the size and density of the text - play an important part in the æsthetic, and in places the new digital versions have erred against the original look of the books (I outline this in more detail here
, as part of another thread).