Because we didn’t always have difficulty levels. In vanilla and BC, you just showed up with the number of people allowed for a raid and did it. Wrath opened it up to have normal and heroic versions of 10 and 25 player raids.
MoP turned that on it’s head by removing raid sizes and then adding flex difficulty. Flex raids were even easier than normal, but they allowed the bosses to scale to support groups of 10 to 30 players. With the WoD pre-patch, they renamed flex to normal, normal to heroic and heroic to mythic.
As far as I can tell, the difficulty of bosses hasn’t really changed since then. “Heroic” raids are just as easy as “normal” was before.
So, yes, the “normal” raids are actually rather new. And yeah, they’re designed for families and friends that aren’t interested in min/maxing everything and spending hours on youtube. They’re designed for people to go in and have a low stress good time, but still get to experience raiding as a group.