Why does WWE change advertised matches?


I wasted another Wednesday night watching Main Event. I was looking forward to Langston/Barrett vs. Miz/Del Rio only to find out that they changed the Main Event AGAIN! Do they just not care about The Main Event or do they figure that people will watch anyways? This is the third of fourth time that they've done this in a couple of months. Also, why do they keep changing stipulations that are added already to PPV matches? i.e. Cena vs. Ryback – Ambulance –> Three Stages of Hell, Survivor Series having Team Punk vs. Team Foley –> Cena vs. Ryback vs. Punk, HHH vs. Lesnar at WrestleMania –> No Holds Barred + HHH's career on the line,. 

Is it just writers' indecisiveness or just laziness?


I know this will come as a shock to the readership of my blog, but WWE's tendency to bait-and-switch their matches is a bit of a pet peeve of mine.  I understand injuries or whatever, but it's WAY too frequent now where, as you noted, they'll advertise something for Main Event on RAW and then just tape a totally different match instead.  Or even the Fandango-Jericho dance contest, where they advertised THAT for a week and then didn't even deliver it!  
It's all Vince, basically.  He's apparently getting more indecisive as he gets older, and the writers just have to go along with what he wants, even if it's changing every two hours.  As it is, RAW these days is getting perilously close to the halcyon days of 1999 where Vince Russo would literally book the shows on a napkin the day of.  Vince frantically rewriting everything for no reason all the time is frighteningly similar.