"We thought of setting the difficulty level about as low as we could go realistically for this game because we saw this as an entry point to the Mario games for a lot of people," director Koichi Hayashida told me during GDC. "So the way we see it is someone would pick up Super Mario 3D Land and play that, and then maybe they would move onto Super Mario Galaxy or Super Mario Sunshine or Super Mario 64 after that."
There's an even more personal motivation. "Speaking as a Mario game fan personally, I should admit that I had trouble trying to clear the first two Super Mario Bros. games," Hayashida said. "So for me, I wanted to create a game that I could at the very least clear. Even if it meant using Assist Blocks, I was able to clear this one, of course."
Hayashida and his team designed a game that allowed "lots of people" to see an ending, which plays at the end of world 8 before the Special Worlds appear. "Even though there's still a little bit of good challenge in worlds 7 and 8, people have at least the option to adjust the difficulty for what feels appropriate to them. If that means they need to challenge themselves to find all the star medals in a certain levels, they can do that. But we're not forcing people to do something at a very high level of difficulty all the way through." At least, not until the Special Worlds.