^^ Because it is a tourist destination.
Nessebar has practically merged with the neighbouring Sv.Vlas, Sunny Beach and Ravda into a town of about 150.000 tourists during the summer season.
While the overconstruction and the crowds have driven many tourists away, it is still among the most visited places on the Bulgarian seaside. It is very diverse too: while Sv. Vlas is more about private villas, yachts, calmness, Sunny Beach is about large hotels, crowds, night life, young people. Nessebar is about old houses, churches and Greek ruins, while Ravda is still underdeveloped, relatively cheap, still has very good beaches and no so many hotels.
Here are some photos of Sv. Vlas:
Those are from Sunny Beach (Slunchev Briag) - hotels and bars everywhere, crowded, no space, noisy...anyway they have very good beaches!
And those are from Ravda, an emerging resort, cheap, chaotic, underdeveloped...yet they have the biggest aquapark there (as compared to others in Sunny Beach and Nessebar):