The reasons I didn't apply to DO schools are:
(1) I would have had to fill out AACOMAS and thus pay a substantial additional fee.
(2) DO secondaries are expensive.
Not anymore so than some of the private MD schools. I'm looking at you $125-automatic-secondary George Washington U. 😡
(3) With DO acceptances, you usually have to accept or withdraw your acceptance within a couple weeks. Therefore, if you get an early DO acceptance and you want to be able to hold that spot, you have to put down a deposit that could be like $500, $1000, or more.
At some it's as high as $2000. 
(4) DO tuition seems expensive to me.
Most of the DO schools have tuition in the $40k range. Barring a state school, that's likely what you'll pay for a "mid-tier" MD school. Some of the priciest schools I've seen are middle of the road MD schools: Tulane ($55k OOS), GW ($49k), Tufts ($53k), Temple ($48k)