League of Legends (and other Mobas) do do thematic variations of their maps. LoL, Dota 2 and HoN all do winter snow versions of their map for Christmas, and Halloween-themed themes for October, etc. The argument of there not being more style variants isn't that the developers are lazy, it's that the map is a very integral part of the Moba genre and that changing the visuals and set pieces around creates a situation where people have to 'learn' each different map's visual quirks and any unique features a reskin has (say, for instance, a wall of terrain that is passable when it's a 'Lava reskin' of the map versus any other variation). From a competitive standpoint (which is what all Mobas clamor to be), it's much preferable to have one set map that doesn't really change and that meta games revolve around.
HotS' model is meant to be less serious and have a bunch of different maps and ulterior objectives. The concept of this is fine, but my problem with HotS is that the maps are tremendously fucking shitty. They infamously had to remove Hanamura from the game for being so bad, and the newest Overwatch map manages to be even worse. Not to mention garbage like Towers of Doom, or perhaps worst of all, maps like Haunted Mines or the Punishers, where the map objective is so bad that it's better to ignore it and shove lanes.