It's easy. Shoaib Malik cannot play pace if his life depended on it. That's a fact. But he's a monster when it comes to hitting spin. Obviously he's much more suited to Asian conditions. Is it really unbelievable that Shoaib Malik's two consequential innings have both come in Asian conditions (one in Abu Dhabi and the other I think in Kandy) and has been hopeless abroad. In England he has an average of 15 and in NZ an average of 12. Averages 41 in Asia.
Similarly Agha Salman is a terrific player of spin and does really well at number 7. But he looks really bad against pace. He may not succeed in English and NZ conditions where pace bowlers bowl 75 overs in a day on relatively seaming wickets. It's a no brainer that he will likely not succeed.
On the other hand, Saud Shakeel looks adept at playing both pace and spin. So he has a better chance of surviving abroad.