I think availability is part of the reason, but I also think that 'better parts' doesn't justify the price increase. They sound similar. They have the same features, they use the same remote codes.
Maybe the power supply is slightly more robust on Integra vs Onkyo, but unless you are really pushing it in a large room with inefficient speakers, it won't matter. Also how many people can realistically tell the difference between Analog Devices dacs (onkyo) and Wolfson dacs (Integra).
I've thought about giving Integra a try, but always end up just buying what I am familiar with - Onkyo.