Perhaps they are attempting to drive passengers away from the trains so they can axe the (apparently little used) Cardiff terminators.
(a) How do you drive passengers away from the Cardiff terminators (which I strongly suspect are a franchise requirement) by providing a competing coach service between Cardiff and Swansea?
(b) The Welsh Assembly Government is, you may have noticed, rather pro-rail. Can you even begin to imagine the sh*tstorm that would emanate from Cardiff Bay at the first little whisper of the Intercity train service from the capital of Wales to London being cut by 50%?
(c) And last but not least, who on earth told you they were little used? I can only assume you've never got on one of them. Don't just drop in unsourced comments like that without some kind of background justification!
Honestly Btline sometimes I do wonder where you dream up this stuff from