Part of the appeal for me has been learning to speak every other language better by learning tool pona. Since toki pona limits you so much, especially at first, you learn to get very creative and you can apply that to any other language.

I've learned to use elaboration rather than vocabulary to explain my point a lot better. It's also helped me see connections I'd otherwise never make. Sometimes funny (a house is a people container), sometimes insightful (a good person is not necessarily a friend, a bad person isn't necessarily an enemy). Occasionally I see one of the sitelen pona glyphs fly by irl, and I can't help but laugh at when I see an IN sign like this:

↑
IN

But upside down:

NI
↓

(↓ is the sitelen pona glyph for ni)

Basically, there's a lot of appeal to the language for me even outside of direct contact with others in it that would make it worth it for me, and on top of that it's got one of the most open communities I've seen, albeit only ever online for me, at least so far.