It's fully possible that the Senju were severely decimated through the Shinobi World Wars, as they would have been on the frontlines (both Hashirama and Tobirama, the two strongest Senjus, died at war). Other Senjus, like presumably Tsunade's parent(s), would have died of old age, and Tsunade was in no real hurry to replenish the genetic line after Dan died.
The Uchiha, functioning as Konoha's police force, would always have maintained a significant presence in the village even during wartime (especially since they were never fully trusted anyway).
Plus, there's no evidence whatsoever that there are any Senjus in the Konoha government aside from Tsunade. The Sarutobi clan was one of the founding clans that allied with the Senju, and it seems like they've replaced the Senju's authority.
Logically, there should be Senju kids running around, but, well, there aren't.
Either way, it's pretty logical to conclude that the World Wars crippled the Senjus as a clan to the point where even if any Senjus exist, they could never have the political significance they used to.
