I assure you it is not a local event. Half a million people do this event yearly and it draws people from all over the country--especially youth.
I think you are referring to the "March For Life" eventevery January in DC. That would seem to be the National event, but there are large marches or rally's throughout the country on or about the same time frame. Texas has a number of large ones.
But where do you get this 85% of attendees are catholic? Maybe in the past, and because your area is heavily catholic. I've read half a dozen articles on the event last month, and nowhere is there any kind of breakdown of catholic or protestant attendance numbers. I'd say there are most probably a nice mix from both doctrines. The speaker platform certainly has a balanced mix, seeing that evangelicals were apparently "officially" invited for what seems to be the first time:
Six surprising changes to the anti-abortion March for Life – CNN Belief Blog - CNN.com Blogs
"Though various religious groups oppose abortion (many support abortion rights as well) the March for Life has come to be considered mainly a Catholic event.
Catholic clergy offer prayers, Catholic politicians make speeches and Catholic school kids fill out the rank-and-file.
Monahan says this year will different.
The March for Life has hired a full-time staffer devoted to bringing more Protestant evangelicals to the protest, and they hope to see that effort bear fruit this Wednesday.
They’ve tapped James Dobson, founder of the evangelical powerhouse ministry Focus on the Family, as a keynote speaker. Dobson and his adopted son, Ryan, will talk about adoption, an issue close to the heart of many evangelicals."
Based on this article, your claim of mostly catholic was probably correct, simply because it was a catholic event, sponsored by your church, run by your church, officiated by your church, etc.
It is what it was, but there are many marches and rallies around the country where the opposite (primarily protestant) is true as well.
It's all good.