Well best reason I can think of is that it's resistant to the action of MAO-A, which does a number on DMT or 5-methoxy DMT, making them inactive orally. As to why it's resistant, rather than just accepting that it is rather open to guesswork, but I think it's likely to have something to do with the possible hydrogen bond between the nitrogen of the side chain and the hydrogen of the 4-hydroxy group. That interaction with the nitrogen atom stops it from being in the right place for the active site of MAO-A, so it doesn't get metabolized.
You'd have to confirm everything above (esp the second bit!), but I don't quite know where at the moment.
As for DET, DPT, DiPT etc. the bigger alkyl groups block the active site of MAO-A, preventing their metabolism. This results in the 5-methoxy being active along with the 4-hydroxy and even the ring unsubstituted parent molecules.
Yeah, I'm pretty certain it'll be all about MAO-A