It would. And as a player if I'm only thinking of myself I'd love to have additional expansions. However that would then raise a problem: you'd be giving a significant advantage to those who complete the event. If it never comes round again then there's no way to remedy this. If it were a event building there'd be no problem. You only have so much space and can simply use a different event building that has a similar or same function. But the more expansions are event exclusive the more you're locking out players from every being able to receive that same benefit - or widening the gap between players that just started and those who have been playing for years in a way that can't be obtained simply from playing the same amount of time unless it's a regular occurrence.How about just one event giving just one expansion as a reward? It would be a great surprise for us.
So the only way around that is if the game checks if you've received an event expansion before and only provides a prize if you haven't won that prize in a previous event. But would you really want to have events where you only get a prize if you've not received the prize in a previous event?