You might be best to ask a Mahayanist. A monk of quite some qualification in Tibetan Buddhism explained it to me a while back I can't quite remember his exact words.
It has a lot to do with their conception of nibbana and what their version of it is about.
He spoke about the bhumi levels in Mahayana, according to them one of the lower bhumi's corresponds to the arahant, and their bodhisattva system goes well beyond that. Since in Theravada a Buddha is a self described arahant, that would mean that our conception of the Buddha is wrong to a Mahayanist. (or more to the point they believe our conception is incomplete) They believe in something about non-stopping nibbana, that the Buddha was enlightened a long time ago and his appearance on Earth was just an emanation i.e. he continues to operate in Samsara.
None of this the Theravada accepts. The Buddha's paranibbana and that of the arahant are in essence identical, once the flame goes out, the flame goes out. Done.
"For a disciple who has conviction in the Teacher's message & lives to penetrate it, what accords with the Dhamma is this:
'The Blessed One is the Teacher, I am a disciple. He is the one who knows, not I." -