MV2 extensions have a lot of API power and it was a common malware vector in the browser since the APIs let you sidestep a ton of regular web security. If you run a popular extension you will get offers to buy the extension which is a nice payday. The buyers would then stuff it full of malware to infect the existing users.
Google makes no money off the Chrome Web Store and their initial attempts to restrict MV2 failed. The goal was for automated approval to suffice. Still, there was certain APIs that required human review.
Google could have continued restricting MV2 until they didn't need human review but they must have got the idea for MV3 at that point. They could also hamstring ad blockers and get some promo packet material.
Google makes no money off the Chrome Web Store and their initial attempts to restrict MV2 failed. The goal was for automated approval to suffice. Still, there was certain APIs that required human review.
Google could have continued restricting MV2 until they didn't need human review but they must have got the idea for MV3 at that point. They could also hamstring ad blockers and get some promo packet material.