Mild hybrids have smaller less powerful motors that aren't integrated into the transmission the way a full hybrid typically is. They can be paired up more easily with different engines and transmissions. While they don't offer the potential for as much electric only running, they do provide electric assist and start stop functionality and at a lower cost. Perhaps if Toyota had a less costly (and somewhat less functional) mild hybrid system in their portfolio, they would be considering diesel hybrids. However, just because we haven't seen something like that yet it doesn't mean we won't. Toyota spends a lot on R&D, much of which is never seen in public until they are good and ready. Toyota certainly isn't shying away from hybrid technology in general.
[Source: Automotive News - Sub. req'd]