All of the above answers are correct. Its a hardware limitation. Windows CE, which WP 7.x is based on, is written for specific hardware. When MS made the switch to WP 8 WP 7.x devices were made incompatible...
To be honest, I prefer WP 7.x to WP 8.x....much better integration and an overall better experience. I understand I'm in the minority but thats my preference.