Is it wrong to cancel events when you don't get the new volunteers for it?
My opinion is no, it's not. Although I might want to evaluate carefully how big the event is and what the bottom line impact is to the group. Cancelling something that brings you $500 of profit shouldn't be treated the same way as something that brings in a $10,000 profit.
Cancelling may draw attention to the issue of a lack of parental involvement, or it may end up just showing you that the event just isn't the best one for the school. Since you indicated that parental involvement is so good in the classrooms, it makes me think that there's something up with the type of events you have going. If it's something you have done for years, maybe people are just sick of it. Or maybe the demographics of the school have changed and it's not as important/relevant to the current group.
A couple of years ago, the school my daughter was in threatened to cancel the Market Day fundraiser. Once they did that, someone who really liked buying Market Day stuff stepped up, because she didn't want to lose it. She ended up being surprised at how much less effort it was to run than she expected, and has been running it successfully now for two years.
Sometimes people assume that there's a lot more involved than there really is. If you know there is someone like in my example above, who has really enjoyed a particular event or fundraiser, but hasn't taken an active role in organizing it, maybe you could sit down with that person and outline the responsibilities (including time estimates, if possible), and specifically ask them to take it on. Sometimes you get lucky and a very direct and personal ask is all it takes.
Good luck!