It is used, particularly with spices and fresh fruit, but sporadically for all kinds of food. As for why it's not universal:
There are some concerns about radiochemical effects, mostly regarding nutritional degradation, with extremely high doses such as might be used to sterilize raw meat for storage at room temperature.
While I can't source this now that I'm looking for it, I vaguely recall reading a survey about food irradiation acceptance; about half the respondents thought that irradiating food made it radioactive. There's much public apprehension, and confusion, with anything involving the word radation.
Similarly with public acceptance, one of the biggest reasons to use it would be to reduce the need for refrigeration. But consumer preference and familiarity makes it difficult to accept things like dairy that isn't refrigerated. UHT milk is available in my country but almost unknown. Even the UHT milk is sold refrigerated! I've suggested it to others before, such as for lack of refrigeration when camping. The reaction was ew, is that safe? Doesn't it need to be refrigerated? Hard to overcome that.