I suspect it’s just because a trial account is just a modified full account. Perhaps the character limit is more deeply coded than we know, and it would be more tedious to adjust it than to leave it as is.
Honestly the thing that would make the most sense to me is that they either compress inactive account data to save space, or the account data isn’t really “that” big of a deal anyway. At a peak of 12 million accounts, but a total created accounts in the 100 million plus range, if the storage space was really that bad we’d be seeing some issues from that.
My guess? It’s just not a big enough deal to do anything about. Particularly given the fact that Vanilla will have basically half of the storage space they do now. Most players will end up with 16 slot bags for the most part, compared to our 30 slot bags. No reagent bank, no transmog, collections, etc.
Character storage space will be pretty lean in Classic vs Modern.