Damon & Elena had a built up for years & that lead to a connection & love more deeper than Stefan & Elena had. And the proof of it is that 6 years after Elena chose Damon & loved him more than she ever thought was possibe.
With Stefan & Elena all happened too quickly that i feel they didn't had a chance to really understand their feelings. I think at the end of the day, they needed each other more than they loved each other.

+ The chemistry is better. The story is better. Their scenes are better. Everything is just better.