One of the hardest routes on El Cap, The Dawn Wall, was done by Tommy Caldwell and Kevin Jorgeson in January 2015. What are some of the reasons to choose to climb such a hard route in winter, when the conditions are probably more adversary?
I just finished Caldwell's book Push. The big walls in Yosemite are essentially vertical deserts. Even in winter they're in the full sun much of the day. It would drop below freezing at night, but during the day temperatures would regularly get above 50 °F (10° C). In the summer the heat can be overwhelming. Aside from the difficulty of performing extremely gymnastic climbing in high temps, sweaty hands make tenuous holds impossible. Gymnastic chalk can only do so much to counteract that. It also turns out that the spring and fall shoulder seasons are problematic because ice forms on the wall every night and peels off during the day. After several close calls with baseball sized hunks of ice, they found it slightly less hazardous during colder periods. On the other hand when it got too cold, the rubber on climbing shoes wouldn't grip as well, and they would just take a rest day on their porta-ledge.
I was there in early January. While the valley was below freezing, upon the first 50 feet climb in elevation after sunrise, the air temperature was very agreeable (50+ °F/10+ °C) with lots of sun. Rejoice for there was no ice underfoot—unlike on the valley floor.
Also, everyone who had been there before were beside themselves with how uncrowded it was. Even with the goverment shutdown, there were almost no visible effects: the visitor station was closed and there were no park rangers around (not even manning the entrance booth). Bus shuttles still ran, trash was removed, all the hotels and restaurants were open and well staffed. (If there were rangers around, would they evict the few dozen tent campers in the closed campgrounds?)
My wife and I hiked to the top of Yosemite Falls. There were perhaps 25 small groups (2-4) of "climbers" on the trail making it to the top. 3/4 of the trail was clear with the last mile or so having packed snow with trailside snow up to a half foot deep. Sunny, gorgeous, clean and crisp air all the way. The overlook at the top was enchanting with a thick carpet of cushy fresh snow and spectacular viewing for about 15 people while I lingered. (I expect it is harder to see in the summer.)
No experience of Yosemite weather, and it sounds like you have your answer, and this isn't wrong, but: people don't climb hard projects just to get their name on the problem. That it's difficult is a big part of the attraction. So long as it isn't wet... there really isn't 'bad weather' for climbing, you just have to factor it into your expectations.