I suspect the semiprotection has a lot to do with it. If our policies were more clear, we wouldn't have a debate every time a high profile event leads to a higher rate of editing. But we do, and a good portion (maybe even a majority) of the time the related article(s) end up protected in some fashion.
This is a problem; high profile articles have a great deal more attention from experienced Wikipedians than most, and also are a great draw for new editors. The no-protection pattern includes lots of quickly reverted vandalism, and a lot of useful new content added by new users that is then molded by veteran editors. The common pattern for protected articles is a steep drop-off in editing (even through semi-protection for autoconfirmed users), preventing the normal bump in article comprehensiveness and quality that should come when a subject hits the news.
I'm sure the other factors are important as well in comparing the two - proximity to large English editing populations, severity, news coverage, etc. I do wish we were more consistent with best practices when it comes to protection, though.
Nathan