I remember reading an article few days ago about this. The gist is:

• Google was not confident with Tensor 1 & 2 chips and didn't want to burden itself with a hardware that was going to be difficult to support.
• Previous iterations of Pixel had Qualcomm chips, and QC doesn't support its chip for that long.

Money. They don't care about selling the older phones.

Feature parity between iPhone and Android is decreasing. Android biggest strengths were cost, features and flexibility.

Google pixel isn't competing in cost. So features and flexibility are what matter. Apple has caught up. Unless you have a very extreme need for flexibility the iPhone looks more and more attractive.

Especially when you consider the iPhone strengths relative to android. Long term software support. Long term hardware support, even through third parties. It's much easier to get someone to replace your iPhone screen and battery than a pixel. When it comes to third party repairs they are often cheaper as parts availability is better long term. These are more important now as new phones aren't getting that much better, so it make more sense to keep your phone for longer.

Because the iPhones get long term software updates, developers target much newer versions of iOS. So apps are better, and are more likely to make use of new features. Many play store apps still target Android 5 from 2014. If your buying an android device apps are developed for an OS version deployed nearly a decade ago.

Much better security and trust in that security. People are increasingly concerned about Google.

Google can't convince people it will be more secure. They can't easily improve third party repair market. So the effective cost per year of safe use of pixel devices is much lower than an iPhone. Google only remedy is longer term support.

If Google cared they would be updating the original Pixel with at least security updates. Apple provided an update for the iPhone 6 this year. The iPhone 6 was released the same year as the original Pixel.