It was probably done for balancing. In earlier games power armor was very rare, and you needed power armor training to use it. In Fallout 4 and 76, power armor is much more common and you don't need training, so they added the fusion core requirement so you can't use power armor all the time.

But lore wise, they always needed fusion cores. It just wasn’t a game play mechanic.

I agree with what @The Appalachian said above -- it was intended as a balancing mechanic. That said, it doesn't succeed very well, because there are enough fusion cores available in-game that with moderate effort and a bobblehead that comes your way pretty early, you can stay in power armor the entire game.

There are so many fusion cores you can essentially use power armor 24/7 in Fallout 4 but im sure balancing was one of the reasons still like Appalachian said.

Early game, they are rare and expensive. Later they are everywhere

Yeah, that makes sense, but I'm not much of a power armor user so I usually just sell the fusion cores