I don't know of any reason but the only reason that there is no iOS 5 update is probably because the performance would be pour, the battery would die faster, and the hardware wouldn't be able to handle it and would be bad for it, the reason the 3rd Gen has it is because it has better hardware to be able to handle the OS, if you are really desperate to get it you can try use an iOS 5 update ipsw file for the 3rd gen and use REDSN0W to update, but it most likely wont work, Im not responsible for data loss or damage.