No takers, huh? Alright, I’ll bite. 
For me, it’s a combination of accessibility, familiarity, and immersion. I do enjoy a pc or console game now and then, but when I play other games I always feel aware that I’m playing a game. Even the most polished and immersive experience is only a short term escape into whatever world I’m experiencing at the time.
With PBBGs though, even ones that don’t look all that great, the medium itself makes a persistent universe somehow feel more real and consequential for the long term. I don’t get this from persistent games on other platforms, and definitely not on browser games that aren’t persistent.
There’s something about this exact mix that feels like an extension of other things that I’m doing online, and therefore inherently more immersive as if the experiences in my online life and my online game life overlap.
I’d love to someday learn more about the psychology around this, but at the very least I can say I that when I play a PBBG seriously, it evokes a commitment that I’ve never experienced with other games. It’s definitely more work, but it’s also more rewarding.
What about you?