#3 Fans remember how good he was as a heel
It is impossible to dispute that Seth Rollins was a much more natural heel (remember when he turned on The Shield and aligned himself with The Authority?) than he is as a babyface. His move-set may be whole a lot flashier now than it was back then, but he was a great 'coward' in the ring as a heel back then, always taking the easy way out to pick up a big win.
Because it did not happen too long ago, fans still remember that run and his feuds with the likes of Kane, Sting, John Cena, Dean Ambrose, etc. back from during that run. If he does go heel now, depending on where he lands up in the draft, he could potentially face off against strong babyfaces like Daniel Bryan, Kevin Owens, Aleister Black, and even Rey Mysterio in some thrilling matches.
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Since they have his heel run as a blueprint (coming off what was probably the best heel turn of the modern era), they simply cannot accept him as a babyface especially if you factor in the way he has been booked. This is why he has heat with WWE fans.
Do you want to see him turn heel again?