Oxford English Dictionary has marked this definition of "here" as obsolete. They describe their rationale for obsolete words as:
If an entry, sense, or lemma is no longer in use in the English language, it may be considered obsolete. This usually means that no evidence for the term can be found in modern English. The latest quotation indicates the period when the term was last in use.
The latest source OED cited of "here" as standard, meaning an army, is from 1508.
The tothir knightis maid care of arthuris here.
They also include one use from 1872 but the use is archaic in context. It is used in a historical essay, knowingly using an antiquated term:
Over 35 men (or 3 × 12) constituted a Here by Ini's laws.
So the expert opinion is that this term is obsolete and no longer in use in Modern English.
Why the term fell out of favor is a complicated question; The English language is fluid, new words are constantly entering the language and antiquated ones get left behind. There are many entries in OED marked as "obsolete," by virtue of the constantly changing nature of the English language.