"Yet schools seem to constantly get rid of these home-grown solutions in favor of Blackboard 'n pals...why?"
Because of CYA support contracts. Executive university ITS staff hate the thought of having the buck stop with their department. By paying outrageous fees to these big players, they always have an out when things don't work - they can yell at someone at Blackboard.
The linked article only mentions Moodle, but Blackboard also announced yesterday that they have hired Charles Severence, one of the founding architects of the Sakai project, in the role of "Chief Sakai Strategist", and also announced that they will provide hosted Moodle and Sakai installations. This is a major foray into the Open Source LMS world, and it's still to be seen whether it is an opportunity to keep relationships with non-Bb schools, or a razed-earth invasion of the OS support arena.
As a side note, technically Banner isn't an LMS, it is a Student Information System (SIS): it goes rather deeper into the registration process than an LMS, and also acts as the HR system at most institutions that use it.