I've been waiting for this game in Gog store so much and now I knew that it will be without my favorite achievements, why? Is it so difficult to add such feature, or what is a reason for that?

This is not the first time when Game in the GoG store does not have Achievements in its version and as I understand not the last... From what I have discovered by communicating with some Devs, GoG has a too complicated process of implementing Achievements to the Game(s). That is why Devs/DevTeams do not implement Achievements in their Games. So there are two ways in this situation: you can buy the Game in the GoG store without Achievements, or you can buy the Game in the Steam store (Epic Games store, Ubisoft store, Origin). At another hand, you also can write to Devs and ask them to add Achievements to the Game in the GoG store, but this may work at best in 10-15% of all cases.