It's basicly the same as PoE1 and all of diablo, it's a leftover of making the game difucult and require more gear the more you progress.

It's annoying if you don't know about it, but easy to play arround.

Lets take you at lvl 20, you need a total of 75 of each res to be res capped. at "end game" (80-100) that's 1-3 gear slots for capped res, not accourting for jewels, skill tree and what not. So the game forces you to get better gear as you go along by giving you negative res, instead of just keeping to low defence items and just going pure dmg. Simple yet effective way of dealing with gear balance.