Yes, because pixels are currently the only android phones with hardware security that third party operating systems can take full advantage of.

Unfortunately yes, there are technical reasons I don't fully understand. I think has to do with how it's locked down, and that it can be relocked after graphine is installed.

Graphine takes security seriously, so leaving the boot loader unlocked is not acceptable to them.

Although, that may change with google removing pixel specific code from the android source.