Pet pens

Because the park does not want pens. We do not allow pens (or dogs being tethered unattended) for numerous reasons. First, pens often do not secure the dogs, they can either jump the pen or dig under the pen. Second, people let their dogs out in the pens and do not clean up after them, either by choice or they just plain missed the fact their dog pooped. Third, People often leave their dogs in the pens unsupervised. Fourth, after a few days, penned dogs often trample the grass down into paths along the perimeter of the fence. Fifth, people often secure the pens by driving spikes into the ground and consequently into the irrigation system. Sixth, pens impede normal maintenance and sometimes are just plain unsightly. It is much easier to say "no pens" than it is to deal individually with each issue that arrives.
As for no pens but crates are OK, I have never seen people that use crates as they would pens, leaving their pets in them for long periods of time unsupervised . And if people put crates outside and leave them, we ask them to move them inside their rig. Have we lost customers because of our dog rules, absolutely. Falls under the "can't please everyone all the time" clause of business.