If memory serves Cubic did the initial RFID readers for BART, after BART finally gave in and joined TransLink/Clipper. They were way slower than the readers every other transit agency used.
Cubic (still) runs and maintains Clipper as a whole, and most other systems like it in the US. They didn’t develop Clipper at first, but have been maintaining/upgrading it for most of its life.
The support/call center is Cubic, most of the readers are built by Cubic, etc.
Right. I don't remember the full rationale as to why Cubic was brought in, but I do remember BART dragging their feet on working with ERG and instead opted to go their own way (EZ Rider).