Another factor is that a lot of liquidity locked up in maternodes means less liquidity bought and sold by speculation on marketplace.
As a whole, I think there is probably just as much liquidity in masternode trading as there is on exchanges. Thats because Dash has a significant number of OTC traders who are in the market for large holdings. They don't purchase those holdings on exchanges.
So even though it looks like the 'reserve market' coin supply is static, it's actually changing hands quite a lot.