lawl. Oh you troll sir, you troll. But let's move on with the obvious. Simple: Support.
As simple as that. IBM will have its support side, but so will Ubuntu. With SMBs and servers, generally there's a couple of Linux techies that know enough not to need support- and thus they get away with simply running CentOS/SuSE/etc without the need for licensing or support.
Mainframes? Ahh. Big dog stuff. IBM provides the low level, hardware support & other backend stuff- whereas Ubuntu provides the software/interaction with hardware side of the support. You think anyone in the place that is using a mainframe might know enough Linux to do the task- however, I would equally wager that this person already has a role and no time to do so
