The few simple reasons as most of the follows replied already which I always see in my company which is one of the WW biggest oilfield service companies are:
- Bad communications so operation team updates maintenance team late.
- Lack of resources making the unavailable asset goes to most important beeded job other than having enough assets to cover all requirements.
- Lack of leadership from maintenance supervisors so they don't plan their pending work properly.
- Equipment damages due to operation where replacement takes longer time to substitute damaged ones
There are many other reasons but from my experience these are the most important ones.