I didn't contribute to this (I already have a Prusa i3 reprap) but my guess would be that it is nearly all about the price. $300 is just about my impulse purch

I didn't contribute to this (I already have a Prusa i3 reprap) but my guess would be that it is nearly all about the price. $300 is just about my impulse purchase price for really sweet "toys", whereas $500+ feels like real money. If I weren't already "into" 3D printing already then something at this price may very well have been what convinced me to jump in just to play around with it.