HoE may change from now to release. It will probably go in the direction of getting harder if anything so if you manage to get an achievement now and someone has to play it when it's 2-3 times harder then it's not fair for anyone who didn't want to buy into early access. If you are really that good, like really, really that good then you can beat it again.
"Rocker, i don't want to have to do normal, hard and suicidal again, i shouldn't have to grind normal and lower levels just to get the achievements!" Well make a suggestion to unlock all lower level achievements when doing a higher level one. I've been checking ideas and suggestions and haven't seen it yet so if it's that big of an issue then sit your butt down and make a thread to suggest it if it bugs you that much.
It's also making sure that if there were any exploits used to get the achievements in early access or bugs that caused an unlock when it should not have been are gone and it's even on release. It may still just be bragging rights but in any case no one like false achievement.
Finally it's once again an even playing field issue. People should have all the same chances to get all the achievements done first if they want it. If you don't care about achievements then it won't effect you. If you do care then there will be more to unlock anyway and lets be honest, beating those 3 maps isn't to much of an achievement in the first place.
As for those claiming "I don't want to have to play for another 30 hours just getting achievements back, i don't have that kind of time". Don't kind yourself, i have seen some of your steam profiles with 1k+ hours on the first killing floor, it won't matter if you play 100 hours now, you will probably play 900 more hours more on full release.