There are two "why did they do it" questions in there. One is about the particular reasons Texas and OU decided to move. Another is about whether or not they made a good decision.
If you're a detached administrator only caring about the bottom line, then hell yes, it was the right decision. If you're a fan, I it absolutely was the 100% wrong decision. However, college sports fans will absolutely willingly eat crap and love it if their AD and the media tell them to.
If you're OU, you've been the big dog in your conference for literally 70 years. Nebraska's best success came when OU had three straight bad coaches, as soon as OU found a good coach again Nebraska and Texas became absolutely second rate citizens. OU got to absolutely destroy your conference comprised of your closest neighbors. Now they're coming into a new conference, predominantly not your neighbors and they aren't going to respect them and they are going to want to wreck their crap. OU goes from running a conference of 70 years to becoming Tennessee and Texas A&M.
Texas already has the biggest budget in college athletics, what the hell do they need more money for. OU ran the football, UT ran the conference, had their own network and everything. Business wise, they are just an also ran in their new conference, no one will placate them or genuflect to them. For football they went from being a clear #2, no matter how overrated they are, to fighting for scraps with Arkansas, A&M, and Ole Miss.