Why isn't "compulse" a word?

As KillingTime noted in the comments, it used to be a word.

A definition from An English expositor: teaching the interpretation of the hardest words used in our language. With sundry explications, descriptions, and discourses by John Bullokar, 1621:

compulse: constraint, enforcement

An example of its use in Anthropologia, or, A philosophic discourse concerning man being the anatomy both of his soul and body : wherein the nature, origin, union, immaterality, immortality, extension, and faculties of the one and the parts, humours, temperaments, complexions, functions, sexes, and ages respecting the other are concisely delineated by Samuel Haworth, 1680:

thus the infant, after its exclusion from the womb, looks on this side and on that side, and in the mid'st of friends sees no friend, and then by its countenance seems to express (or at least it is legible in its face) what aristotle, when among his friends, vocally pronounced, in non-latin alphabet, o friends no friend! and because it can not express its own sentiments, or manifest its wants in the unknown language of that strange land, which it is now by force and compulse reduced to inhabit, it begins to utter its complaint in its own proper tongue; and bewails its forlorn condition with ehu's; and such like deplorable interjections; but at last finding a necessity of its abiding here, it endeavours to acquaint it self with the modes and fashions of the place wherein it is, and to imitate their customs and practices

It may have been in use in the 19th century still. The following was spoken by Dr Charles Tanner in the House of Commons in 1890 (taken from the HANSARD corpus):

Now, Mr: Courtney, I do not desire to continue this debate: I only got up for the simple purpose of trying, if possible, to spur on, to egg on, to compulse, to coerce, and to exercise some pressure on the Attorney General, who seems content to sit dumb under the charge which has been brought against the Government from this side of the House