For one, because they (HW & Tekin) are marketed towards racers primarily, Castle for one came out as an upgrade for Traxxas and Similar brushed motor systems and such... They were and are not really focused on any sort of racing that one can perceive. I see more of the Brand Reps (Tekin, Hobbywing, Orion and such) at races, actually racing for one, and listening to feedback from the racers using their products. Also consider that the varieties of actual race oriented products offered by HW and Tekin outnumber what Castle has currently.
Personally I never saw them (Castle) as any option for a racing vehicle, 1/8 or 1/10. I ran LRP, Novak (until they closed shop) and now HW. All brands that, in my curiosity, I saw people also running at the track, and they worked quite well. I can;t speak as to Tekin, yet the CS from HW has been awesome, and their product has always worked for me (may be different for others). From what I have seen on here, Tekin has representation on RCtech, and they do a great job tending to their customers here. One thing too, is that on track one gets support from many that are running a similar product.
This is of course all my experience and opinion, others may chime in with their 10 cents as well.