I can think of a few reasons:
Code Review is still in its early days.
People are concentrated on the main site, don't expect much from secondary aspects of the site. If one thing is worrying is that not many people seem to participate on Meta (as the site is still shaping). Chat, I don't care about.
Code Review overlaps (in expertise & interests) with very popular sites.
Mainly StackOverflow and Programmers. So most of the crowd here is active on one or both of these sites (and others), so people naturally lurk those sites' chat rooms.
Another thing is that several extended and off topic discussions that may arise from a Code Review would be perfect as questions on those sites. I did a review recently where the OP asked me for some clarifications on Dependency Inversion. I pointed him to Programmers, as a better place to ask. He read a couple of related questions there and problem solved, didn't even had to ask a new question. Why waste any time on chat, when most relevant discussions will be perfectly on topic on one of the other sites?
Code Review is somewhat discussion oriented.
Think about it. You post a piece of code, and people critique it. Why would that discussion benefit from chat? If you want a deeper analysis, just ask for it (either on comments or with a new question) and let people harvest the sweet rep.
Chat is evil.
There I said it. The StackOverflow rooms especially are full of noise. I'm all for mindless fun, but that is not why most of us are here.
Chat will kick off, naturally, when the site grows a bit older.
As a sidenote, as we are both PHP developers: The Stack Overflow PHP chat room is not a very welcoming place. If such behaviours migrate here from there, I'll do everything I can to fight them off. I can't do much, obviously, other than flag, flag, flag, and then some. And handle chat flags aggressively.