Well' I cant speak for everyone else here, but I think that at least Gokaigers popularity with the fansub watchers is because of Power Rangers.
I might be assuming a lot here, but I dont feel its unfair to assume that most people who come into Sentai/Kamen Rider subs are more familiar with Power Rangers than they are either of the franchises. So you have Decade and Gokaiger and both of them are throwing around names that mean very little to you if you're not versed in the universes yet.
So take Decade, which first off is using alternate character interpretations of characters that fansubbers might not be familiar with in the first place (unless I'm mistaken, not every series Decade visits has been fully subbed). So you arent as invested in the characters. The fight scenes are cool, and Decade is what made me go download Kuuga and give Ryuki another shot, but when I watched it I had no basis for who these characters were beyond "That ones Dragon Knight and the other ones Masked Rider".
Then Gokaiger, which was my first fansubbed series. Again, I had no idea who these guys were, but because the guest appearances are the same characters that they were in their series, the plots a little easier to follow. They tell you everything that you need to know about the plots of the guys who guest appear "Gai Ikari was a badass lancer who was in Jetman, and he died" or "A guy named Joe dealing with dealing with the struggles of a friend who has been transformed and serves the enemy, We've done this plot point before, lets have Joh meet Joe". Also, as a Power Ranger fan, even if I had no idea who the teams were that were appearing this week, I could still follow the Gokaichanges, because I had some connection with these characters, even if it was just with their suits. When Gai makes "Go-Onwings", I didnt have any knowledge of Go-Onger yet, but I knew Gem and Gemma, and loved the scene anyways because I had some context to work with.
Also, yeah, Decades plot was a mess (Do we seriously not know what Narutakis deal was?)
Anyways, in summary, Gokaigers *might be* more popular in the fansub community because they have the Power Ranger suits, and at that point its not a matter of who did what first its a matter of Nostalgia.
and of course, you know, Pirates. Everythings better with Pirates.