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I have always wondered about this, with desmodromic valves they do not even have to cope with compressing valve springs so they do not have a big longitudinal load on the belts.
I think it may be because the pulleys are so small when compared to a car engine (although most car engines use the difference in size of the pulleys to get the half enine speed needed for the camshafts so the crankshaft pulley is often not that much biggger than a Ducati one) so the belt has quite a small radius to get around which maybe causes the belts to delaminate internally and therefore have a shorter recommended life.
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