Why go through all that effort to create an interesting setting only to just throw all in the trash it will just be a huge waste, 1% of the galaxy has been explored so there are plenty of new stories to be told, changing the galaxy will just feel like they jumped the shark (which they did with LP, the giant T-800 human reaper).
Your objection here makes very little sense to me. ME1 explored the Citadel, Noveria, Feros, Therum, Ilos, Virmire and a number of various optional worlds which far outnumbered the plot worlds. ME2 explored Omega, the Wards, Illium, Tuchanka, several brief exposures to plot-driven missions on new worlds, space stations, and all of which were far outnumbered by the optional worlds - also almost all new. ME3 let you explore the Citadel, a number of plot-driven missions on new/old worlds and a number of optional missions on new/old worlds which far outnumbered the required missions.
The point is, over the trilogy and in each game you see a brand new setting for the MAJORITY of the story. There are only a few recurring places that you visit over and over - most notably the Citadel.
So what are you complaining about? A game set with the samespecies in a new galaxy would have a familiar setting in that you would be surrounded by familiar faces and tech, but it would likely be far outweighed by the number of brand new planets, space stations, and species that you would encounter.
Just like...you know...every Mass Effect game thus far. The only difference is that there would be no Citadel, no Omega, no Illium - replaced by all new vistas. And that's a good thing. A very good thing. This is a new story, leave the old to rest and move on. It isn't the Citadel, or Illium, or Omega, or Earth, etc. that makes Mass Effect what it is.