Because I think Pixy is much harder to use than Arduino.
Can you explain a bit more as to why you think Pixy is more difficult to use than Arduino?
In order to use Pixy you don’t need to write a single line of code for it (although you can if you want). You just need to hook it up to a powered Arduino and teach it to find the objects you want. Then information about the objects it finds will be sent to Arduino over SPI, which you can access through Pixy libraries we’ll provide. So really, all you need to program is an Arduino, which is great because they’re easy to use 
Scott