That have different reasons, but the most popular reasons are probably:
1.) Popularity
Verses like One Piece, Naruto, etc. are really popular and have a big impact with a bigger fanbase. Those fans outnumbers mostly other verse debaters, but that is not only based on the HST. Verses like Dragonball and OPM at this point are even similar.
2.) Versatility
The HST verses contain a lot battle capable characters, while in a lot other/less known verses the number of battle capable characters is smaller.
3.) Knowledge
Primary knowledge about the more popular verses, less interest/knowledge in other verses make debating often very monotonous.
That also is based on my personal experience as someone who debatted around 15 years, because i debated only Naruto and One Piece when i start with it.
Later i gained more knowledge about a lot other verses, and the HST become more and more boring/irrelevant for me, because if you debate one and the same verse over years, you not only get out of ideas, but also out of interest to debate it.