What is the capitalization rule for acronyms? E.g., why "radar" and not "RADAR" but "NATO" and not "nato"?
I think this has a lot to do with the fact that certain acronyms were intended as (or have become) the general words for certain things, and others are actual names for things.
"NATO", for instance, is an abbreviation for the formal name of an organization; as such, it seems obvious that you have to leave it in all caps.
"Radar" and other words such as "laser", on the other hand, are invented words for newly invented phenomena. The fact that they come from acronyms is almost beside the point, since they could have been instead named by inventing Latinate or Greekish words (such as *collimolux or *syngrammophos (yuck on both counts)) as sometimes happens in science. They are simply coinages and thus bear only an etymological relationship to the original acronym, and therefore we use them as "normal" words, which includes dropping the all-caps on them.
The word anacronym is relevant here.
an acronym that few remember what the letters stand for, such as laser and radar
Generally, once a word becomes so frequently used that its acronymic etymology is less relevant, the capitalization is dropped.