LogFAQs > #905874205

The games expanded Mega Man's(and thus the player's) toolset. The first Mega Man game gives you, like, three good weapons, two useless weapons, and one situational weapon.
Mega Man 2 gives you Metal Blades, which are better than all of those combined(they're broken good, of course), but virtually all of the other weapons are good or at least have a use. And, oh yeah, there's now eight of them instead of six!
Mega Man 3 gives you the slide, which drastically increases your mobility and dodging capabilities, as well as featuring another really good set of weapons(only Spark Shot is bad).
Mega Man 4 gives you the charged shot, which enables you to deliver powerful attacks without even needing to rely on boss master weapons, and it STILL gives you some excellent tools to use.

Compare that to Mega Man X, where you can dash, climb walls, get eight robot master weapons, and all of your weapons can be charged. Your toolbox is comparatively huge.
Although it's true that this caused the games to become easier, it (more importantly) made them more fun to play.