BG1&2 used AD&D 2e, whereas BG3 is using D&D 5e. In addition to being separated by multiple editions (3 or 4 depending on how you could 3.5e), these editions were created by entirely different companies.
BG1&2 were developed by BioWare, whereas BG3 is being developed by Larian Studios.
3.5 was just a revision of 3, it can't really be counted as new edition, just as upcoming "D&D One" can't be as well.
Shame though, 5e already 9 years old. Would make sense invest into 6th edition, instead of trying to patch quite broken 5th one.
