Sunny Deol started his career as a romantic hero and hit jackpot with his very first film Betaab, which earned 6.75 crores and became the second highest grosser of 1983. But 80s was a particularly good time for romantic films with proper romantic actors like Kumar Gaurav struggling after the initial success and Rishi Kapoor doing supporting roles and heroine centric films. Rajesh Khanna lost his glory as well and Jeetendra moved to Masala films. Sunny Deol also couldn't keep it up with romantic image intact
His films after Betaab were Sohni Mahiwal (Above Average grosser 3.20 crores), Sunny (Disaster), Manzil Manzil (Flop), Zabardast (Flop). All these movies were romantic or drama genre and these films didn't help Sunny at all. And then hos career was revived by Arjun (1985) which got him much acclaim and success and established him in rough and tough roles.
Sunny struggled to give hits between 1986-89 when his films were hit and miss. He had big hits like Paap ki Duniya, Tridev, Chalbaaz and moderate success like Vardi. But he also had major failures like Saveray Wali gadi, Samundar, Sultanate, Dacait, Yateem, Joshilay, etc. Every time Sunny's career hit rock bottom he was revived by the action mass masala genre. Ghayal made him a top star, Ghayal and Damini got him National Awards, Ghatak, Border, Jeet and Ziddi made him top of the game.
But there was an issue. His non action films stopped working. People expected Sunny to be all dialog baazi and action. Dillagi failed, Ye Raaste hai Pyar ke failed, Imtihan failed, Ajay Failed and bis atyempt at comedy in Jo Bole So Nihal failed badly. Also his films started looking like the same. When Ziddi became a major hit they made Salaakhein which was very similar to Ziddi but despite being a decent film it failed.
Another problem was his relation with makers. Sunny Deol was a difficult person to deal with. Many of the makers who made hit films with him eventually broke away due to his unprofessional behavior. Rajkumar Santoshi had Ghayal, Damini and Ghatak with Sunny (his career defining films) had a fallout with him, Dharmesh and Sunil Darshan worked with him in Lootere and Ajay but then had a fallout during Janwar due to money misappropriation by Sunny. JP Dutta had a fallout with him after Border, he had a fallout with Yash Chopra after Darr. And the makers he had good rapport with included Rahul Rawail who did half a dozen movies with him but Rahul Rawail lost his mojo after stupendous hits in the beginning. With Sunny he gave Betaab and Arjun in the beginning but then Samundar, Dacait, Jo Bole So Nihaal all failed and Arjun Pandit did average business. Similarly his success with Anil Sharma was on and off. They had the super successful Gadar franchise but Anil Sharma delievers many duds after every hit. So Gadar was followed by Hero, Apne was followed by Singh Sahib the Great
Last thing, Sunny Deol is himself obsessed with his action image so he couldn't do more than one dramatic roles at a time. After the failure of Shaheed where he played Chandresehkhar Azad he refused to do period films. Now after the failure of Batwara 1947 i guess he will again refuse period dramas. Mohalla Assi failed, Ikka didnt get good response and Sunny altered the genre of his film Safar (which was supposed to be an emotional film rmake of Maratha film Pravaas) to Gabru to suit his macho image. So i don't think Sunny doesn't remain committed to other genres when they don't yield results.