Why was it so hard?
For every reason.
Those are 16px gaps, which means you have a one frame margin of error when landing between the gate pairs. Worse yet, your align with respect to the 16px gaps changes with every single 16px gap. This constantly screws up your visual cues for knowing when you have the safe landing pixels.
Oh, and see that shit at the bottom of the screen? Yeah, you have to do gate jumps in the middle of a screen transition, over and over again.
It's probably the hardest screen in the game, which is impressive given some of the other absurd stuff in the 80s and 90s. I don't think you can really understand it until you play it for yourself, preferably with the context of having played the rest of the game before it.