Here's why:
* Imperative sentences give commands or instructions. They typically start with a verb and don't have a subject (it's implied to be "you").
* Adverbs of frequency (like "once") tell how often something happens. They are typically placed before the main verb in a sentence.
In "Come here once," the adverb "once" is placed after the verb "come." This is grammatically incorrect in an imperative sentence.
Here's a grammatically correct way to say the sentence:
* Come here once. (This is still a bit awkward, but it's grammatically correct.)
* Come here one time. (This is a more natural way to express the same meaning.)