They are probably an experienced team members that are needed on ME5, and management decided that they can’t distract them. And these new devs would spend a lot of time figuring out a completely unfamiliar codebase (also creating a proper remaster would also necessitate a lot of low-level changes to the engine which will make it much harder). So it’s a business decision, not technical one.

One has absolutely no connection with another. It’s been 13 years since BioWare last used Origins’ engine. Few people in software development stay in one company for that long (especially programmers who would feel no personal connection to company’s IP, unlike writers or artists).