In almost all the Mahayana sutras, the audience of Buddha listening to the discourse consists mostly of Bodhisattvas who appear magically from different lands and even Buddha-lands. For example, in Lankavatara Sutra Buddha explains various Mahayana concepts to those Bodhisattvas. What is the usual level of attainment of these Bodhisattvas? If they are already so advanced as to appear in front of Buddha to listen to discourse, why do they ask amateur questions?
The same applies to Pali canon in which Buddha's audience is almost exclusively comprised of Arhats. If Arhats are already liberated, why do they need any further teachings?
The only exception seems to be the Heart Sutra where a Bodhisattva discourses to an Arhat (Sariputra).
I am not sure that your two basic assumptions are correct, ie. that in most
Mahayana sutras, the audience is bodhisattvas (from buddha lands, etc.), and that in the Pali canon the audience is almost exclusively
In the Samyuktagama, for instance (I take this example as I've just finished reading the whole thing), the over whelming impression is that the target audience is in fact not
arhats, but regular people, non-buddhists, or unawakened monks and nuns. Sometimes, some arhat or another may be present, but not very often (even Ananda wasn't an arhat during the Buddha's life).
Mahayana sutras also often contain a very mixed audience, including the human and non-human. It may require more than the single example of the Lanka to argue your case. I can easily find at least a few dozen Mahayana sutras where the target audience are arhats (and others), eg. Taisho 310 Ratnakuta sutras 6, 9, 21, 28, 33; Taisho 320, 331, 339, 341, 347, 357, etc. etc. Just a few off the cuff.
Your counter example of the Heart sutra as "the only exception" just maybe shows that you haven't looked at very many Mahayana sutras at all. In general, the whole basic Prajnaparamita genre (for the first 500 years or so, at least) is also directed at arhats, with only cameos from Maitreya.
So, I'm not sure about your basic assumptions here, which underlie your questions.