Subgenres in dance music are _extremely_ fluid and change from year to year and even month to month. Some of them are basically just adjective + genre.
The reason dance music has so many sub-genres has to do with how djs mix records. They want to mix songs _smoothly_ and to do that they need songs that sound substantially similar to other songs they want to play. So they find a song or two they like, then they go looking for songs with similar tempos, energy levels, instruments, etc, so they can mix them together. If enough people settle into a particular musical territory, that becomes a "sub-genre", at least for now.
Even something sort of broad like "techno" evolves a lot over time.
The reason dance music has so many sub-genres has to do with how djs mix records. They want to mix songs _smoothly_ and to do that they need songs that sound substantially similar to other songs they want to play. So they find a song or two they like, then they go looking for songs with similar tempos, energy levels, instruments, etc, so they can mix them together. If enough people settle into a particular musical territory, that becomes a "sub-genre", at least for now.
Even something sort of broad like "techno" evolves a lot over time.