I think this decision has a lot of different reasoning behind it. So in no particular order...
No reasonable source of more Leo2s to upgrade to get a uniform force.
Buy new Leo2s from the Germans? Get screwed on the price, no significant industrial bump, and it would still take as long or longer than the M1s. (No, no industrial bump from the Abrams deal either, they're in a hurry - see below)
Time. Leo2s would take longer to deliver in significant numbers and the best of them is arguably not at the level of the very latest M1s. (as long as the Poles are getting the newest armor package and ammunition) The MGCS isn't even planned to start deliveries until 2035 - 13 years after Poland plans to start taking delivery of the Abrams.
A very satisfying thumb in the eye of the Germans and the French (who doesn't like that?). Ignored by them even though the Polish market is at least as large as the planned combined fleets of those countries.
Diversity of suppliers. Despite the logistical challenges, changing political winds are less likely to cut you off from two sources of arms than from one.
The Wilk project doesn't seem on it's way to put tracks on the ground anytime soon.
More trust in the US/Polish relationship than the one that they have with the Germans (historically and currently). Also, a desire to cement that relationship even tighter.
The M1A2C is in production, it's real, it's having the final bugs worked out now. The MGCS is currently electrons and paper. Let's count how many multinational armor projects have failed to produce usable hardware. No, let's make it much easier and count how many have not failed. MGCS may be the best tank ever, it may never actually exist at all. Similar issues with the K2PL (though to a much lesser degree) a new, so far unbuilt, version of a design that still has at least a few issues.
Poland wants to replace most or all of their Soviet derived tank fleet. You might disagree on the need, but it's not your call. Replace the T-72s, then the upgraded T-72s, then the PT-91's - now your tank fleet is down to two types, both Western.
I'm sure I will think of more. That's what I have for now. Worth every penny you paid for it...