Grad xfer is a completely different beast (and juniors are often grad transfers as well as seniors.) Grad transfers are usually not planning basketball careers but professional careers, and the grad school choice is often not available or not as good at their original school, so they are actually shopping for an academic program that playing basketball will pay for.
And with this past year being a 'free year' in terms of eligibility a lot of graduating athletes have a unique opportunity to get a free head start on a masters degree with their fifth playing year. And that will be true for 3 more years.
Uconn with 5.5 freshman this year, a class of 3.5 freshman next year, and 2 more committed for the following year - all really good players - probably isn't shopping. Most coaches preference is to coach players 4 years, and getting a 2 year player is frustrating as there is a learning curve and then leave just as they really start to get it. With no sit out year, you at least get to start right away and are less likely to have a good transfer bolt a year early for a pro career.