Think of it from the advertisers perspective. Mobile apps tend to bombard users with adverts and as an advertiser, you wouldn’t want to pay for every single impression shown to a single user. They watched the first instance and didn’t buy into your product, so further shows are a bit of a waste really unless your campaign is focusing on embedding a brand name in anticipation of a future sale. It makes much more sense as an advertiser to only pay for actual installs, assuming you’re advertising an app that is. The ad network can only pay you a cut if they’ve earned something from the advertiser too, so if the advertiser is paying per install, that’s where you’ll get your cut.
Networks that still offer PPC or PPImp need to entice advertisers away from the PPInst networks and the only way to do that is to offer really cheap campaigns, so I imagine your cut from PPImp advertisers is a lot less than you’d get from a PPInst network.
That said, I’m inexperienced with mobile ad networks myself. Just commenting from the tiny result-sets I’ve seen so far. I’ve watched the market change over the years with web too though - PPC just isn’t what it used to be.