Well it's pure nostalgia bait.
Many people like X1 despite never playing it, because they were told it's good and they feel like they have to like it to be hip or what have you.
Only X4 has that phenomenon worse, because it was pretty mediocre and also the beginning of the downward spiral.
I feel like anyone who has any interest in the Mega Man franchise has played X1 by now, either through a port or the original through emulation.
X1 was a really strong first entry to the X series. All of the Maverick designs are fun both in design and gameplay, the soundtrack is easily the best of the 16 bit MM era, and the new mechanics were implemented well. I'd say that X1 was the most accessible (easy) game in the 16 bit era as well, so it's an easy point for people new to the series to dip their toes in.
X4 was the first 32 bit entry and Capcom went all out with the spritework and sound. Plus, it was the first game where Zero was a separate playable character outside of X3's swap-in mechanic where you'd lose access to him if you died while playing him. Just about every MM-inspired game that comes out these days usually has both an X character and a Zero character that specialize in ranged or melee.
The X series started going downhill when Kenji Inafune starting putting his OC (Zero) front and center of the story. It's especially bad once you hit
X6 X5 through X7. It's only made worse when you consider the Mega Man Zero series canonically takes place in the same timeline, where Zero pretty much completely replaces X by the end of events. Don't get me wrong, I enjoy the gameplay of the Zero series and I like Zero as a character, they just did my boy X dirty.
Interestly enough, Inafune apparently didn't learn his lesson from his time with the MM franchise, because he pulled the same shit at Inti Creates. GV from Azure Striker Gunvolt has been reduced to a powerup/mercy mode in the latest entry, and the main character is, surprise, a sword user with a red color scheme and a dash mechanic.