Because the company providing that service can't index every network on earth right when it first starts. If you're using a newer wifi network, it might not be in their database yet. Either that or they haven't added the networks in your area into their system.
I am finding that this only works for open networks ... or my home password protected network. I can find plenty of secure networks, but I can't get a location position off of these. Also, I can't get it to spit out directions for me. It gives me a route not found message.
yeah, ive had the same problem. me and my friend were in this coffee shop the other day and he had his iPhone and i had my Touch out and he went on maps and found his location but when i tried, it said "Your location could not be determined." my location finder has yet to work.