The usual suspect is lack of time but it is not the only one. Others might be: no any tangible results, it is just boring, the same things are being discussed over and over again, the same format year in year out, it is an arena just for a few loud and talkative ones, and inevitable conflicts sometimes.
As retro facilitators we have a lot of responsibilities in helping teams to overcome those challenges and to understand the value of retros. The teams that have experienced really good retros with impactful results after are much less reluctant to having regular retrospectives.
What might be problems with retrospectives? Those are the most typical ones that I have observed – or have been part/reason of 😉