It’s likely to do with part 3, but from the manufacturing side. It’s cheaper to run production of one type of thing than two types of things. Further, if it’s cheaper to produce just SATA3 instead of both SATA2 and SATA3 then it makes the availability of SATA2 go away, so while we may suppose SATA3 is overkill or more expensive it may be that in practice you simply can’t buy a SATA2 controller. While SATA2*should* be cheaper, that presupposes it’s a viable option.