Because there ARE not more transactions in the Dash network. In fact on average it's even lower than 0.7 tx/sec.
Even Bitcoin (which has about 200 times our market-cap, and has a LOT more merchants who use it) has only 2-3 transactions per second.

BTW, 0.7 TX/sec means about 60000 transactions per day. For a currency where most investors just hold their coins that's actually quite good.