I love Mal (I know, I don’t talk a lot about it…) but I mostly love him post 1960 and even more after 1964 when his sound changed.
I think the reason why he is on so many Prestige recordings was a practical thing. He lived quite near the studios and was able to compose 3 or 4 songs in 24 hours. Mal said in interviews Bob Weinstock used to call him, told him who were going to record the next day, then Mal wrote a few pieces with those players in mind and bring his work to the studios the next day.
Contracted and avaible: he was probably a logical pick as a pianist. Especially for Prestige, whose records always feel more like sessions than albums. Wasn’t Weinstocks whole philosophy: get some musicians, get some compositions, try to record everything in one take, pay the musicians and see if it sells?