They were reimagining the songs and adding new elements - him jumping throw a window on Smooth Criminal, coming out of a spider in Thriller, 3D thriller, the burning bed in Dirty Diana, the plane in Man in the Mirror, rearrangement of Dangerous.
Die hard fans seem to think these shows were only about themselves. The vast majority of people had not seen a Michael Jackson concert LIVE so there was no point him coming in with some obscure tracklisting of songs most people don't know. It may be your 50th concert, but to most fans, it's their first. Most people going to those shows don't know Invincible and it wasn't even Michael's place to force it on them.
Seriously though, could you imagine being a casual Michael Jackson fan who knows the 10 most popular songs he's done, sitting there listening to Invincible. It would have been quite perplexing. You also would have probably given the show bad reviews. Many die hard fans don't live in the real world and don't have the ability to even empathize with most normal people. This Is It was about bringing Michael's best hits to the stage and that is exactly what it was shaping up to do.
Could he have snuck a few surprises in? Sure. But when you've got a set time limit (2 hours I believe), you want to make the most of that time. You don't need to spend 20 minutes of that confusing half of your audience.
Make no mistake, This Is It would have been an absolute "vibe" (as the kids would say), from start to finish. You would have walked away from the show feeling like you'd seen something truly special.