One problem is the wide disparity in attention levels at different sites. Is 39 moves the longest a chess game can go moving only pawns?Is 39 moves the longest a chess game can go moving only pawns? was posted to Math. I think it is a much better fit for Puzzling, but the activity level at Puzzling is so much lower. Cross-posting would help in cases like this.
The proliferation of sites (particularly betas) makes this disparity a frequently recurrent issue and puts the growing Stack Exchange network somewhat at odds with a strict policy against cross-posting, especially in the long run as topic overlap increases.
Some wiggle room could help release a lot of this tension. Consider supporting related feature requests (see the comments for many more) and encouraging conscientious cross-posting practices (see @Gilles' answer here and Shog9's here) rather than simple discouragement. If done right, it could help build nascent communities and collaboration among them. To connote more positivity to the productive practice with an alternate buzzword, one could think of it as cross-pollination.