KoTOR II was developed by an entirely different studio, Obsidian. The reason there weren't consistent teams in game was that there weren't consistent teams in real life. But regardless of development politics, the two games explored completely different aspects of Star Wars (BioWare took a more traditional route, and Obsidian a more nuanced one), so the near total replacement of the crew was warranted and quite necessary.
But to your point, I disagree entirely. For the most part, I loved the way BioWare handled the team transitions throughout the series. Not only did the new crew members give a new and interesting view to the universe, the old crew wasn't forced to stay static. Writers don't always foresee exactly where the plot will go and what precisely what themes they'll want to touch on (BioWare's writers more than most), so a dynamic squad gives them just a bit more (and much needed) creative freedom. They wouldn't need to be restricted to the same old perspectives or writing for a cast of ever expanding personalities. The gameplay would benefit as well, letting the designers insert totally new classes and powers without lore restrictions.
More importantly, I think seeing our old friends in higher places commanding a team of their own gives the game a real sense of literal and emotional progression. The real world doesn't cater to us, and neither should Mass Effect's. Sometimes friends get a better job opportunity, have a family emergency, or just have to leave, and I think future Mass Effects should explore the emotional impacts of those changes. I stand by my belief that Mass Effect's narrative is the greatest trilogy ever achieved by games (despite it's major blunders), because it wasn't afraid to move the world without you.
I won't say that BioWare has to change up the squad each time, but there are distinct benefits to it.
That said, I don't want BioWare to forge those who move out of your immediate circle of friends. I hope all could get a treatment similar to Liara's or at the very least, Mordin's or Wrex's.