My first thought was that it was formed from well + come (past participle of come), so it wouldn't make sense to take a past participle and form a past tense from it--past tenses are formed from infinitives, not participles.
But then I actually looked it up in the OED, and it looks like I was wrong: it comes from will + cuma (meaning "comer" or "guest"). But because of the influence of the Old French bien venu, meaning "well come," we came to see will as well and cuma as come, the imperative or infinitive form of the verb.
So anyway, because it's a noun that turned into a verb, it's conjugated as a weak verb (-ed ending) instead of a strong verb (verbal ablaut).