A lot of players, maybe even most players, like repetition and turtling.
1: Repetition: Knowing that they can use the same build orders over and over again on the same map with events unfolding in mostly the same way is more comfortable than trying to navigate unfamiliar territory.
2: Turtling: There is not a lot of harassment of the core base on Dual Gap and similar maps. People have a much better chance to move up to big economies and higher tech levels (big expensive T3 and T4 toys) without dying to mass T1 spam.
It's the same reasons that some people play the campaign missions over and over again.
You get repetition and turtling on Dual Gap because of the shape of the map. Depending on how a map is shaped, that can make some strategies (e.g. raiding, early t1 spam, gun comm pushes, hang back and build T3 arty) more or less viable.
Not everyone wants these things. I'm bored by campaign missions and I don't do much gapping. But it's not hard for me to understand why other people enjoy it.