The short answer is because development cycles can vary.
The longer answer has a lot of explanation, but I’ll try to focus on the beefier challenges. Diablo III is supported on PC, Mac, and all current generation consoles. That’s five separate release clients and four network platforms (three of which are external partners) that need to be tested and sometimes re-generated and re-submitted if there are any issues. Localization also takes time, as we’re supported in 13 different languages. We avoid patch cycles occurring during active Seasons so that any game changes that happen, even if they’re just gameplay optimizations, don’t impact players who compete on the Leaderboards.
We still, generally, target an estimated 3 months per Season (and we’ve stuck pretty close to that, though last patch had some unexpected issues that required us to push the Season back). Ultimately if we have to make a choice between a Season running a little long or getting the extra time to make sure changes are thoroughly tested and the patch is as high quality as possible, we’re going to err on the latter.