[irondragon]

Yep. That's right as SideswipePrime says, in that Go-Busters had the signature city.

I think maybe why it's not as regular in Sentai as Rider though, is that it's more powerful thematically when a single hero takes it upon himself alone to nobly defend a city. When a team decides to do so, the impact is lessened a bit.

It's like how a lot of Riders are semi-tragic heroes because they have to take a huge burden on themselves, while Sentai teams have each other to form a strong whole/sum of parts. So while Haruto in Wizard, for instance, had to be the sole "final hope", this wouldn't be necessarily as powerful a statement if a team of good friends had to fill that role.

So, I guess, along those lines, having a team defend a city might feel more like any Police/local law enforcement/military organisation (which some Sentais have been, to be fair) and being part of some unit that's designed to protect a place is sooort of less striking than one hero that's lost it all, still deciding to do the right thing. Like Shotaro in W losing his parents and his mentor, but still loving the city.