He's made appearances on all of these shows because of how well he knows Trump. And also because he's possibly the only celebrity willing to support him.
Never underestimate the power of Ego either. Being invited on shows like Marr, Sunday Politics and Question Time (and to a degree Peston - though that's linked to his employer) because of his expertise (real or perceived) on a subtject gives him a sense of importance, and definitely will be an ego boost.
It's worth reminding ourselves that Piers was on a cable news channel (which don't rate amazingly, and which he was let go from - actuallly for the right reasons ISTR. He stood on principles in taking on the gun lobby...) in the US, and is on a little-watched morning show here. He was sacked by the Daily Mirror, for publishing fabricated photos too, don't forget. He's not amazingly credible in some areas...
It's not like he's David Dimbleby, Huw Edwards or Graham Norton. He's not got that level of credibility or that size of audience these days - so being invited on shows that are the leaders in what they do (rather than the also-rans he often presents) is presumably quite ego-massaging.
(Life Stories was probably the biggest thing he's done here ?)