Assuming your group operates as an open membership group with regular general meetings, I would take your petition and a few people willing to speak in favor of your events to your first PTO meeting of the year (or the last one for this year, if it's not too late).
Clearly outline the benefits of these activities and honestly ask for the leadership's reasons for canceling them.
Give them a heads-up that you will be doing this, because that is the right thing to do.
Do not be confrontational. Be concerned that your group is not acting in the best interests of the students and the school.
Our 'Santa Shop' was cancelled in 2004 because we had no volunteers to run it. We asked personally, plead at meetings, and published in our newsletter that if noone agreed to run it we would cancel it. We did.
There were a good many upset parents. We had no idea how popular this thing was. Last year, we managed to talk someone into chairing it, and we are happy we did.
Parents love that they don't have to drag their kids through the mall at holiday time to shop. There is also an educational element -- divvying up their funds to get something for everyone on the list. Kids also feel special giving an item they selected secretly.
Brainstorm and list all the positive things that come from your events. Don't forget about benefits to the volunteer community and tradition.
Here's a link to the (spartan) web page for ours:
Cape PTO Holiday Shoppe
If your group is a school committee that is now controlled by the principal, you may need to do this at a BoE meeting. Non-confrontational, concerned, and prepared is the way to go here, too.