Currently there is either no demand for more tx (outside of "stress test" blocks are no more than 30 to 45% full since January), or some other factor is limiting the demand.
Interestingly enough, orphan blocks are rarer now than they used to be, but SPV mining, 1 tx block and partially filled blocks despite a significant tx log are more common.
Maybe blocks are already too large compared to the propagation & orphan risk? miners limiting effective block size below 1 MB to minimize their risk of orphans.
Remember that the mining relay network is used by pretty much every miner now, you put yourself at a disadvantage not to. This makes a significant contribution to the low orphan rate on the blockchain these days.
