With even more teams having a shot at the playoffs, more teams are going to go for it at the deadline. Nowadays, there are more teams in the race than not. So now you have maybe 18-20 teams going for it and trying to find players to put them over the top from the remaining teams not in it, because they can't just go get someone in FA, and those other teams in the playoff hunt aren't going to give up anything, especially pitching. During the offseason, everyone gets a chance to trade with everyone else and the slate is clean. There's also FA where it's just about money. No one is getting desperate because they find themselves either dropping after a good start and they want to stop the bleeding, or are surprisingly in it and want to make sure they can stay there.