Man what is wrong with you people. The SMB engine was so great that they didn't feel the need to change it. And you have to actually get good at the levels to do well.
Super Mario Bros. had tighter controls than most other Mario games, such as Super Mario World. I'm not too fond of SMB's controls. And yes, I am good at Super Mario Bros. 2(J) and I've beaten the whole game many times, but it's just not recognizable as a whole new Mario game. To do well in any game, you have to be good at the game. That is the case with all Mario games. Along with that, you might have to try a few of the levels over and over again to beat them. That too is the case with all Mario games. Super Mario Bros. isn't the only game like that.
Somewhere they figured out that if they made easy games instead, they could make people buy more of them, and we end up with a bunch of weakling gamers.
That's hardly the case. Super Mario Bros. is just as easy as Super Mario Bros. 3 and Super Mario World. If making easy games made people buy more of them, and if Super Mario Bros. is as hard as you claim it to be, then explain why Super Mario Bros. is the best selling video game in history.