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To your point, the non-Sony Blu-Ray studios appear to be adopting the VC1 or AVC path; Sony seems the only one stuck on Mpeg2...
That's not entirely true. Despite all their HD DVDs being VC1 (save for U2 Rattle & Hum which was MPEG4), all of Paramount's Bluray releases have been MPEG2. According to comparison reports, they look identical, so really much of it comes down to the competence of those doing the encoding I think, and not just the codec choice itself. That said, although the likes of Paramount appear to be able to get their MPEG2 encodes on par with those of their VC1 encodes, no studio has really proven MPEG2 can do anything better than VC1, so, to me at least, it would still appear a pointless waste of space to keep on using it.

The first handful of Warner Bluray releases had to be MPEG2 for a technical reason, but they are now all VC1 (they use the exact same encodes now for both formats). Fox and Disney meanwhile are inconsistent, siding with MPEG2 for some titles and MPEG4 for others. Quite what the two of them use to decide on using one codec over another on certain titles I am not entirely sure.

Weinstein Co titles on HD DVD are all MPEG4.