Its mainly that 155 and 185 are pretty stacked divisions, and 170 and 205 are less stacked. So you spend years getting up the ranks in 155 or 185, you'll never get a title shot almost no matter what, look at khabib who had at least a 9 fight ufc win streak ? before finally getting a title shot, and ferguson who is now 12 with no title shot. They can go up a class, and if they get a lucky good win they can propel up the ranks like a new guy who's on fire and "unbeaten" at that class.
Sometimes its a good move to go up. Rumble Johnson somehow was fighting at 170. So hungry and energyless, dehydrated from weight cut you can't perform optimally. It depends how much weight you cut to make weight and how rangy you are. Some guys just do better not cutting much weight because they can eat, and your brain is the last thing to rehydrate so people are easier to knock out cutting more weight. But that is mostly genetics, it didn't help rockholds chin lol.