A lot of people are after specific tail types.. Everyone has their own likes and dislikes.. Many love halfmoons and plakat tail types...
Good form, which is uncommon to be found in pet store bettas. Typically found in well bred bettas, coming from GOOD breeders who breed show quality fish..
Pet store wise people seem to be attracted to solid whites, blacks, etc/ Marbles are also liked by many. Red and blue are dominant traits and therefore more commonly found in bettas, so they are less sought after. veil tails are very common as well, so less people admire their beauty.