Interesting question. I did some poking around the language notes archive and I discovered that this decision was made on the 13th of October, 1999, but the notes do not give a justification for the decision.
Off the top of my head I don’t see any theoretical or practical reason why we could not have field-like explicitly implemented events. Nor do I see any reason why we particularly need to. This may have to remain one of the mysteries of the unknown.