This is...like...the eternal question.
I've wondered about this before too...scattered thoughts below that may not make sense.
I've done Princess twice - once in 2011 and a second time last year. In 2011, it was totally different - the race was newer and smaller (I think that was the 3rd year they had it? And there was something like 14K participants), there was no 10K/challenge component, I don't remember hearing/seeing anything about corral jumping, cheating, etc. (doesn't mean that it didn't happen, of course), and I personally don't remember etiquette being that bad. runDisney had just become "runDisney" (formerly the Disney Endurance Series I think) so their social media presence wasn't what it is now. Not as many people really knew about the races back then.
Obviously a few things have changed since then.
- I think the biggest thing is that runDisney's popularity greatly increased, and they began to add more races and to expand the capacity of the races they already offered. Example: In 2013 I did the WDW Half for the first time - exact same course as Princess - but the field was suddenly HUGE at 22K participants. That one remains my worst-ever runDisney race experience because I expected it to be like I'd remembered at Princess (frolicking through the course with tons of room and minimal photo lines), but it was incredibly crowded and I remember barely being able to run down Main Street, even though I started in the same/similar corral. Princess eventually grew to be that big, too. They made the corralling system better (it used to be 6 corrals starting every 10 minutes...then lots more corrals that were smaller starting every 2-5 minutes...now, whatever they're doing, which doesn't necessarily seem better?), but going from 14K to 22K was just insane.
- Princess seems to be their most consistently popular, best-selling race weekend - it was, I think, the first to add the 10K/challenge, and I think it's widely regarded as the most-welcoming, least intimidating (I guess?) weekend for newer runners. You will see a lot of first-timers at Princess (myself included, it was my first-ever half marathon). And newer runners just don't know race etiquette (again, I'll include 2011 Me in that...I didn't know what I was doing). runDisney publishes guidelines in their event guides, but they can't make people read them.
- I think social media plays a big role. People have runDisney blogs, Instagram accounts, etc. They want hits on their sites and head pats/congrats for getting a medal (notice I said "getting a medal," not "finishing the race" - looking at you, cheaters and corral jumpers!) or posting an awesome race/medal picture (see the previous post where people were taking selfies in the middle of Main Street). I think the mindset of "I paid for this race, I'm getting my picture/medal/etc." is pretty big at princess...I hate to say it, but it's a sort of sense of entitlement that I just don't see at the other race weekends.
- Along with the "look at me!" aspect of social media, I think another thing is that thanks to the internet, it's now widely known that what runDisney publishes on their website in regards to sweeping, who gets medals, etc. doesn't really line up with what they actually do. You don't HAVE to keep a 16:00/mile pace to avoid getting swept, they'll still give you your medals if you're over 16:00/mile and you finish ahead of the sweepers, and it's possible to cheat the corral placement system (see corral jumpers - I know someone who cheated with her corral placement, allowing her to start in the front half a couple of years ago, and she finished in something like 4:30 - still was allowed to finish, still got the half/GSC medals). You don't HAVE to finish the race to get a medal. And so on. I don't think this was as widely known a few years ago compared to now...so you have people that just want the medal/shirts/kudos, so they cheat. It just didn't used to be like that.
And to be clear, I do think that this stuff happens at ALL of the race weekends, it's just the most prevalent at Princess.
EDIT: I was bracing for the worst last year but thanks to decent corral placements and purposely going to the expo late in the day to avoid the nutso crowds, I had a pretty good experience.