Lots of reasons: It can be hard to get stuff that requires extended concentration during the business-as-usual part of the year. An uninterrupted morning is va

Lots of reasons: It can be hard to get stuff that requires extended concentration during the business-as-usual part of the year. An uninterrupted morning is vastly more useful than six scattered half-hour blocks between meetings and seminars and helping students. The summer is actually fairly productive for most people for the same reason; very few people are actually off (as in at the beach); they're just not teaching. There's a brief period of calm between the end of the academic year (which is chaotic with exams, end-of-year stuff, and holiday parties too) and the beginning of the "personal holidays." If you've been mulling something over, now is a great time to finish it off. Finally, it's nice to start the year with less stuff on your to-do list. I don't think anyone expects more favorable peer review over the holidays; in fact, our lab lore suggests reviewers are grumpier now. In any case, stuff submitted in mid December probably won't even go out for review until next year.