Is there any reason not to do market/vendor 2-of-2 in place of centralized? think about it, both of them you will lose your coins, if they want to run the moneys gone either way, but what 2-of-2 offers is a security against wanting to run in order to steal the central escrow, instead if they run (to save their asses) they will get nothing, no multimillion dollar bonus reward.
Is this better than 2-of-3, no of course not! But you got to rememeber 2-of-3 is higher barrier to entry for buyers and less likely to be used.
So whats the excuse EIC and others?
Caveat-Emptor : 2-of-3 is a good trust-marketing tool for building up their centralized retirement fund.
I think their reasoning is likely to strike a balance and offer both something familiar but not run the risk of sharing the fate of every multi-sig only market to date. Mr. Lee's Greater HK looked promising and was multi-sig only, they vanished without a trace, for the most recent example.
The impetus to offer centralized escrow or not is on the vendors themselves because it is not required, vendors can choose to offer only multi-sig in their shops. Maybe once they get more traffic and business they can focus on further optimizing the available options once people have gotten used to the new waters, so to speak.