Cost -- it is way cheaper to use IPR and avoid discovery associated with the other factors that happen at trial. Speed, the PTO is generally faster.

It's not really about cost for most people who really like the IPR. It's a way to get a second chance to invalidate a patent and a way to drag out litigation. The cost of an IPR is actually not that much less than the cost of invalidation during a trial (although it saves you on other discovery because you can often stay the trial during your IPR), but it's a second invalidation path you can take at the same time.

The reality is that you get multiple bites at the apple. You challenge at trial and in an IPR. Lose both, challenge with an EPR and appeal the trial. IPRs were initially created to simplify trial and cost, but once the estoppel provisions were determined not to have much teeth, it just became something you did because there was no downside.