Yes, we were asked this at the Atlassian Summit 2 weeks ago, where we were Diamond sponsor (top), Lucidchart a gold sponsor and Gliffy didn't even have a booth...
I understand the psychology, I'd say there are 3 factors to explain this:
All three of us have integrations outside of the Atlassian ecosystem, but our sole commercial focus is within the ecosystem, i.e. we make all our money here, unlike the other two. Our target is to own the Confluence Server diagramming market and we're priced to do exactly that.
Our pricing didn't use to be that far off our competitors, but others have increased their pricing, notably Gliffy increased their pricing somewhat last year. At the same time Gliffy has failed to innovate in this space for some years now, their product looks very stagnant. How is this benefiting the customer? It's not. We say go all out to delight the customer and focus on improving the product constantly, plus keep the price the same. A product's value for money should increase, increasing the prices with new features isn't our ethos.
Efficiency. We're a distributed team over 6 countries with less than 10 full-time equivalents. Gliffy has about 30 staff in a nice San Francisco office and Lucidchart 200+ employees in a VC funded company. If you find yourself saying you prefer draw.io over the other 2 options, surely the question is what on earth are the over 20/180 employees doing that we are doing better with 8 or 9 people?
So, please just focus on the product, our rate of innovation and our presence and commitment to the ecosystem. If you must ask the pricing question, please ask "why are the other products so expensive?" :).