I think they fear more unstable server with higher breeding. I can't explain it to myself any other way. Since incubator and maewing, players are breeding more and more. I guess we don't see any higher breeding rates on weekend evo events anymore.
They could it alternate, one week breeding, on other weeks the other rates.
@ Wildcard: You could at least communicate that why you no longer switch on a breeding event. Not to mention is very bad style.