It's not really that the pancreas itself can't handle it (though this may be true also). What happens is that having high blood glucose, even if it's not continuously high, is just bad for you by virtue of the high concentration of glucose in your blood. Having high blood glucose leads to the creation of "Advanced Glycation Endproducts" (AGEs) which bind to certain receptors (RAGEs). The AGE-RAGE interaction is linked to downstream diabetes side effects like atherosclerosis.