Because the hardware was designed to run Jellybean. Updating to Lollipop is pushing the hardware more than it can go. Reflash Jellybean to it and you should be fine. (You'll have to unlock the bootloader, and install ADB and Fastboot Tool on a PC - and unlocking the bootloader will wipe the phone, so back up everything [see Backing up an Android Device].) The ROM is at Google's site. Use nakasig for the model with mobile data or nakasi for the WiFi version.) That will get you back to where the hardware was designed to run. If you want anything later, you're just going to have to buy newer hardware.