I can think of a few possible reasons, in addition to what's been said.
The longer, sunnier days in summer could give an extra chemical kick of the energy and motivation that's needed in order to go through with a ctb.
Warmer weather, for many, is simply more comfortable, and preferable to being in a cold environment during one's already-cold final moments.
It's easier to get around, if you're looking for an outdoor ctb. No snow in the way, trees are fully alive and strong to hang from, and if you're trying to drown, the water will be much less icy.