Otherwise they'd have to let the actor go and hope he'd come back in a couple seasons, or re-cast him. And there's a chance viewers would forget about him. I mean, they did the same thing with Jaime Lannister in season two, giving him extra scenes that weren't in the book and bringing some of the third book scenes forward so he'd have things to do and didn't have to skip a season.

Basically, we know what happens to Theon in the intervening time, so why not show it and keep the actor?

My gripe is that they're showing Thoros without Dondarrion. His actor is great, so I won't complain too much, but it sort of ruins the idea that Thoros is sticking close to Dondarrion to keep him alive as his own personal version of his religion's savior, like Melisandre with Stannis. But maybe they're not doing that anymore.