Here's some of the factors that come into play.
A key one: Shatner has lived through lean times in the past (post Trek he ended up doing TV commercials for margarine)… And so when the work comes, he grabs it and does it. Those lean times makes you appreciate the work when it comes. And makes you hungry for more.
Second, he's a driven personality. I've known people who works with the Shat and when he works he works really really hard. He gives it "all he's got, Captain." He and I share that – the idea of "relaxing"… well, other people do that. He can't stop working. He'll stop when he's dead.
Third, he's a pro. He shows up, hits his marks, does his lines, and will draw an audience just because of his history and what he has done. The industry knows that and they respect professionalism (as well as the "draw an audience" part).
Finally, he has changed over time. Once a leading man, he has transitioned into a character actor as well as (as needed and when appropriate) a semi-characture of himself… he does what he thinks is necessary to keep working, to keep moving forward. He's a smart talented man.