Besides security policy, mDNS uses a link-local multicast address, so routers will not forward it to other network segments. Many operators utilize a number of VLANs on their wireless networks, so this creates an issue when devices in separate VLANs attempt to discover each other over mDNS.

This is usually solved through some sort of mDNS proxy, but you don't really want to proxy everything. If your phone discovered all of the apple tvs and chromecasts across campus, then it wouldn't make for a good user experience for anybody.