Guys, when you see questions like this, the answer is almost always "...because we couldn't be bothered" or "...because we didn't have the resources/time." But that's not an especially inspiring thing to say in an interview, so they try to save face. If the team really wanted to include an RPG mode, they would include an RPG mode.
It's much like with Star Fox 64 3D - when an interviewer asked them why there was no online multiplayer, they said they wished to focus on the local multiplayer, which they then claimed was original and had a lot of work put into it. Essentially, it's the same kind of nonsense answer. What they actually mean, but are not willing to say, is that they didn't want to. Those features just aren't a priority for them.
For my part, I'm not especially gutted about the lack of RPG elements. It's still up in the air whether I pick up Mario Tennis Open (might wait until it drops to £20), though I will commend Camelot for pushing it forward in other areas.