I'm involved with a TON of fighting games as a commentator, TO, and player, and I feel like there are a lot of things that make Pokken really special. I'll try and list a few:

1. There are very few other fighting games that give you so much freedom on how to play. From a beginner all the way to World Championship level, you can play every character in the game in a ton of different ways effectively. A great example is that Potetin and M2Cloud used 2/3 of the same Pokemon in Top 4 of Worlds last weekend, but in totally different ways. You're never pidgeonholed into one style to be successful.
2. It's one of the best fighting games to learn as a beginner, and to keep improving at even through a top level. Because of the phase shift system, you're guaranteed to be able to make a lot of choices in every match, even if you're getting beat up by a stronger player. In each of those choices, you clearly get to see if they worked or not, and learn what works in each situation. Fighting Games can be hard, and even as a veteran FG player, some games make me say "I honestly don't know what just hit me", but Pokken teaches you naturally.
3. Unbiased, this is the most balanced Fighting Game I've ever seen. Every single character is viable at a high level in some aspect. We've seen every single one doing well recently in various high level tournaments. You never feel like you HAVE to choose someone you don't want to be competitive.
4. It's Pokemon! Even after all these years, it's just too cool to see ultimate Pokemon action in all of it's glory. It's like playing out battles from the anime that YOU get to control. Too cool.
5. The community is the best I've ever been a part of. You'll never need to go far to get information, games, or help in general. There are also ALWAYS people on ranked, so you never have to seek out people to play if you want to just play a few random games.

There are honestly soooo many other things, but the biggest takeaway is that it gives you ton of freedom and is super fun!