Why is the trail head closed off of pinnacle hill Rd. by 55th ave?

Erosion

I'm assuming you mean Pinnacle Peak Rd. I don't 100% know why it is closed off. But I do know that after several years of people hiking those trails it wears the trails down and erodes them. Every so often, maybe every 10 years or so, the city park rangers will permanently close a trail and realign a new one close by the original trail. So it may be closed for maintenance at this time.