As I understand it, the actors signed multi-year contracts. I'm not anywhere close to an expert on Hollywood legal documents, but I know something is going on now where Jennifer Lawrence is obligated to do the X-Men: First Class sequel, which is messing up the scheduling of filming the Hunger Games sequel, and I remember Patrick Stewart talking in an interview about signing a many-year contract but everyone assuring him that the show wouldn't last that long and he had nothing to worry about.
Perhaps it's just that Beltran signed a 7-year contract, and wasn't willing to go through the costly legal negotiations to break it. Shrug.
There's that. And then, frankly, there's the fact that it is so incredibly difficult to make a living as an actor that I in no way would begrudge an actor who does a television series just for the money. That's the sort of money that, if you're careful, can basically set you up to live a middle class lifestyle for the rest of your life, even if you never work again. For people who struggle to get by in their chosen profession, that's nothing to sneeze at.
(Not saying Beltran did it for the money -- I'm saying I wouldn't blame him if he did.)
And even if the characterisation and plotting are weak, it's still hard work. Long days, continual pressure, and - in some cases - stifling costumes and make-up. This is to say nothing of the publicity (which is undoubtedly exhausting).
I'm not suggesting television work is like hard labour. But it certainly takes dedication and a certain amount of perseverance.