There are a few possibilities here:

1) To start off, he may have been born with a weak constitution, so he may have more genetic problems than others. Growing up, he may have been exposed to a lot of wear & tear; may not been brought up with nutritious food and proper ways to take care of himself.  So by the time he started on working to transform his mind & character, the rate of entropy has accelerated to the point where his cultivation might not have been strong enough to halt the entropy.

2) He may have had difficulty surrendering and letting go of some deep attachments. This can cause major leakage of energy that is seriously needed to repair his physical and genetic problems.

3) He may have had difficulty in fully understanding the teachings, and after that to apply/practise the teachings in all areas of his life.  This could have been caused by misconceptions or deeply ingrained notions that created mental locks which he may not have recognized due to low energy potency.