Because their Presidents decided to go against the advice of BY and think they can get the eastern expansion when the ACC implodes and they will pick up Syracuse, Pittsburgh, Louisville, Duke, etc
It is a risky play as I think establishing the NE presence now with UConn would have given them an advantage over the ACC when the time comes and I think this western only move with only two new western states gives the ACC a better chance of out-surviving the B12. But I guess only time will tell if that is the case.
But UConn is more valuable than Syracuse, Pittsburgh, or Louisville. Yes, if the ACC implodes they may be able to get Duke and some others who fall shy of the B1G or SEC, but they can't afford to wait until 2036 to expand.
The bottom line is UConn is more valuable than the average current B12 member, so UConn raises the per-school value of the conference. So it makes sense to grow.
I think it's more about who to pair with us, and landing on a good number of schools, and geography / travel costs / identity. If the B12 is committed to going East then UConn will be in. If they want to avoid the East, or need an even number of schools, then it may get tricky.