Why can some babies not breastfeed?

Many reasons. For example, babies born prematurely (< 34 weeks) can be neurologically immature and have trouble with suck and swallow. Various neurologic problems can cause trouble with all motor activities, not just breastfeeding. An anatomical problem such as cleft lip or palate or tongue-tie can also interfere with proper suck and swallow. These are just a few examples.