Interesting question, Pompadour. The short answer is...just because.
There is a slightly longer answer, of course. I used to use a pyragon as my primary of choice for the last, let's say year and a half. This semester is the first time I used the praxis.
A little back story. My pyragon started acting funny at the beginning of last fall semester. Whenever I attempted to slamfire, every other disc will jam, preventing a continuance flow of shots. Eventually, I got tired of trying to fix it, and I bought a Praxis. Alongside that, I have never been a big supporter of slamfire. It has its uses, but for the most part I prefer single-fire mode (I hate wasteful ammo bursts from slamfire).
The Praxis, as far as I knew, had a pretty good track record, it didn't have slamfire (so it couldn't jam from trying to do so), and it was cheaper to buy than another Pyragon (I was broke at the time
; this is where "just because" comes in). So essentially, the Praxis fulfilled all of my prerequisites for my battle rifle concept (see my original post for an overview of that concept). My reasoning at the time was that I needed another blaster, but since I really didn't use slamfire the Praxis was good enough to do the job.
I will admit; the Pyragon IS better than the Praxis on paper. That being said, the Praxis is good enough to get the job done. I have yet to experience reliability issues from it, it fits my playing style (single-fire mode), and it has done a pretty damn good job so far.
So basically, to answer your question, there is no reason for me to buy another Pyragon, even though it is better than the Praxis. I'm just trying to save money (and space....so many Nerf blasters) by keeping the Praxis.