I think that those elements that they added that SMB1 didn't have before are things that can be more easily overlooked, but slopes go against SMB1's visual identity in a much stronger way. I think it's a mix of both that and that a slope tool was difficult to make as simply as the rest of the tools. The game already makes upsidedown slopes in the airship theme so obviously it's not impossible.
I suppose another reason might be that slopes only really fit the plains and underground themes and they didn't want to create slope assets for theme where few people would use them and they'd look bad anyway.
Really, a slope tool can be as simple as the slopes generated in the airship theme. Just have a sloped terrain tool, and when you paint with it it generates slopes along the outer tiles automatically.
To use my earlier example in reverse, if you painted this with the sloped terrain tool:
http://i.imgur.com/BOsheYP.png
you'd get this:
http://i.imgur.com/o3PLiCt.png
So nix that. A slope tool would be very simple.
They could just say that SMB1 Mario doesn't have the ability to slide and/or make slopes not exist in SMB1 style.