Because in the past you didn't had analog stick.
Not good enough reason. There are LOTS of things in SMB that weren't there in the past. E.g. short levels with limitless restarts, free selection of levels and other things. It's a perfectly modern arcade game, it's simply stylized a bit like old ones, that's all.
Why isn't there a way to smoothly control meat boy's moving speed with the keyboard? There should be no advantage to using the controller.
There SHOULD. Otherwise all this stuff about 'controller isn't necessary but neither is bathing', etc. is just pointless blabbing. In most modern PC games that were originally developed for consoles the advantages of gamepad are absolutely obvious (say, one of my favourites, Assassin's Creed, but it's true for modern arcades too), while in SMB it's by far not.
Yes, I just realised this a minute ago. I knew you could change the .cfg file, but thought it didn't work.
I wrote it in the first post btw. Yeah, when I used keyboard I changed that to Z for acceleration and Shift for jump (like in Mario Forever), that's much better.