Why can't all these GoT actors act well in anything besides GoT?

I think you're overgeneralising.

Charles Dance has shown plenty of acting chops in other films and TV shows. I forget the name of the actress but she is Arlya Stark and was pretty good in the Dr Who episodes she was. I think the problem is with the projects that they are choosing. This movie and Terminator Genisys were pretty poor for dialogue. The actors weren't given much to work with and possibly they're not as well directed.

Rose Leslie was horrible in GoT.

I thought Rose Leslie was great. 

Took me 10 min to figure out that GoT is Games of Thrones or something... probably.
You know not everybody watches that awful series.

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Lincoln Lee: I lost a partner.
Peter Bishop: I lost a universe!

It's because most of them are not good actors to start with. There are some amazing ones in GOT, like Charles Dance, but most are quite... hmm... not so good. They fit in their roles and that's about the only thing they can do well enough. From now on, they will be doing the same thing over and over and over wherever they go in productions with not much value, capitalising o their GoT fame. Emilia Clarke is poor in every single thing she did including GoT, Richard Madden did Klondike acting as himself I think, Jon Snow will forever remain Jon Snow- I saw Kit Harrington in Pompeii and it was so bad that we kind of laughed the whole way through. He also did a play o West End and from what I can remember, his acting was not really praised as amazing in the reviews.

In the beginning there was nothing, and it exploded.

Emilia Clarke is bad in GoT too, one of the worst ones from the main cast in fact. I actually thought Leslie was alright as Ygritte, but I haven't seen her in much else besides.

'Rose Leslie is bloody awful in this...' Are you serious? Rose rocks as Chloe!