Because Clarke is a luxury in Memphis that they don't really need with JJJ, Adams, and Xavier Tillman there. Crowder can come in and play the 3 or 4 while Clarke is a 4 or a 5 and they have 2 guys ahead of him on the depth chart and Tillman about equal as well. With Clarke being on the last year before he hits free agency, they don't really need him. They could use a playoff tested vet in Crowder who can bring toughness and experience. Plus they save almost $3 million, at least, in salary this year by making that move.
It's not a huge upgrade but Memphis is really young still and could benefit from a guy like Crowder.
I'm not saying it's a slam dunk but it's a fair deal that I don't think either team could say they won unless pics were involved and then the team who got those would win, basically. Memphis doesnt really need Clarke though. Jae can play SF, which Clarke can't, and they don't need Clarke at Center or PF now, really. They'll likely let him walk in free agency after this year so why not try and change things up a little and see if Crowder can improve things there now.
Also Jae helps Memphis vs Golden State more than Clarke does.