You have to prove that you're making good product, otherwise the industry won't just let you have solo release for no reason.
Remember, SRK got Diwali and Xmas solo in 2011. Then both those movies failed to deliver on the expectations. So he had to face a clash in 2012 and a partial clash in 2013. After CE did so well, he again got Diwali 2014 solo. But once again he gave a bad product. Since then he hasn't given even a half good movie (except maybe Raees), so why wouldn't others try to clash?
Akki's Rustom also had a clash, but he proved that he has the product. So now he's getting solo. And even Varun has some confidence as of now.
If SRK gives a good movie, he'll definitely be getting a solo release next time out.