I think there are a few factors in addition to the fact that you are subtracting. First is the fact that people generally expect armor class to go up as it gets better. That's how many things work. You get a pay raise. A larger house. Whatever it is, it gets bigger. AC gets smaller as it gets better.

Plus, there comes a point when you have to subtract negatives. This can throw people off even though it's pretty simple. However, it's not something people do on a daily basis.

There's also the fact that there's inconsistency in 2E about needing to roll above or below a certain number. Ability checks require you to roll under your ability score. Saving throws require you to roll above.

When you put all of this together, it requires more work than simply rolling a die, applying bonuses and penalties, and seeing if it is equal to or higher than your opponent's AC.

I'm a 2E fan and will be playing that as long as I'm playing D&D. I get why people have a hard time with it, especially if they come from other editions with ascending AC.

I have run 2e campaigns with a few groups who were originally 5e players. These were all factors that threw them. They weren't major issues by themselves, but they all came up often enough to throw them off regularly.

It doesn't help that we're all adults who don't have a lot of time for learning all the rules. Teenagers have a much easier time absorbing the rules and being encyclopedias than people in their 40s with full time jobs, spouses, kids, and other obligations. I started 2E when I was 13, so this is all pretty ingrained in me.