They had and have a large/enterprise customer sales culture. There were few SMB sales for IBM back in TWSr's days (how small a mainframe do you want? Still costs a mint.)[1] and now that IBM has SMB product to sell, they aren't interested in the building out a sales org to address that market[2]. So they've done what almost everyone else has done and outsourced that to a channel.

[1] Yes I know IBM used to sell everything down to the pens and pencils, but virtually always to support some very expensive other purchase. [2] Yes I know IBM has things called "SMB sales" or some variation. From my experience at IBM, they were either targeting some specific product/market combo, or they were a bad joke; not exactly the A-team. YMMV.