.

yeah basically games lacked depth back then

I know one thing, when I beat Battle Toads it truly felt like I had just accomplished something pretty significant.

It's a combination of several factors, many of which have already been named, but I think there are 2 main factors.

1. Game design was, in many ways, in its infancy. What's considered hard or unfair now was just accepted without much question because both developers and players didn't really know any better. (Admittedly, part of this "acceptance" was also that there simply wasn't as many games to play as there are now.)

2. Playtesting wasn't done nearly to the extent it is done now.

Let's look at Megaman 2 as a quick example of this. First, let's start with Quick Man's stage with the instant death lasers. They come out with no warning or foreshadowing for the player to anticipate them and they kill you in one hit. This would be seen as total BS by any modern player but I think most players just thought, "Wow! This is hard." And kept trying until they got it or realized they could use Flash Man's weapon to make it easier.

Another example is Heat Man's stage with the long sequence of disappearing / reappearing blocks at the end of the stage. About half way through this gauntlet if you miss a single jump you fall to instant death. Most people through trial and error (and frustration) eventually learned to just use the jet item to get past it but there is no hint that this is the best way to do this. This type of challenge wouldn't make it through play testing now days. Now days this would either be flat out told to the player or the challenge would be gated entirely until you had the right item to bypass it easily.

Less focus on story and more focus on gameplay.

Because people weren't spoiled pussies back then. We now live in the everybody gets a trophy days...to even include getting one for finishing pussy arse tutorials for the games made today.