Bulgarians go to Sandanski for a variety of reasons,it's quite a popular base for Melnik,Petrich and of course Sandanski. Also a lot of Bulgarians are now travelling across the border to go shopping in Serres and Promahonas,using Sandanski as a base.
It's very popular with Bulgarians,especially from Sofia,and Greeks to rent an apartment for the weekend rather than to stay in a Hotel. The following site is widely used by Bulgarians so is worth a look.
in Bulgarian ; "Pochivka.bg" :http://pochivka.bg
or in English ; "Bgstay.com" :http://bgstay.com/
Most of this site is aimed at Bulgarians so is mostly in Bulgarian language.
If you do a search for apartments in Spa Resorts, you'll see that Sandanski has by far the most selection (77 Apartments).