There is a simple reason, Sallu's superman image is not even a decade old, while SRK's romance image has been there for atleast 22 years now. Before that Sallu was Prem for a while, then in limbo for a while, then he rediscovered himself.
SRK hasn't found a new image, and he hasn't needed to. If romance is done well, he can still excel in it.
Ultimately it comes down to the type of movies.
Just look at WOM. Dabangg best of 2010, Ready best of 2011, Bajrangi Bhaijan best of 2015. Quality just hasn't been there for him, that's the main issue. When he did come back to full fledged romance, it was the worst movie of the year, so...