The then-WWF sure loved rhyming, or near-rhyming. Over the previous year, MTV aired WWE specials such as, "The Brawl to End it All", and "The War to Settle the Score." Those were fine for televised one-offs, but an event with higher ambition needed a bolder moniker.
Credit for the "WrestleMania" name is often given to Howard Finkel, but former booker George Scott claims to have come up with it. According to Scott, six months before WrestleMania took place, a meeting was held to determine the name. Somebody in the room said "Mania", and Scott logically put "Wrestle" in front of it. Somebody else said Colossal Tussle, and it won a hasty vote. The next day, per Scott's story, he all but cornered McMahon, and was able to sell him on the stronger "WrestleMania" name.
For what it's worth, in September 1985, a Madison Square Garden event entitled, "The Colossal Jostle" took place, with Andre the Giant headlining against King Kong Bundy.