In Terminator 2, why didn't they just send out a T-1000 to protect John Connor?

A T-800 was sent to protect john connor. A T-1000 was sent to kill him. A T-1000 is stronger and more advanced than a T-800. Why didn't they send a T-1000 to protect John Connor instead of a weaker T-800? Also, in Terminator 3, a T-X was sent to kill John Connor. Another T-800 was sent to protect him. T-800 was obsolete by then. The T-800 itself called itself an obsolete model. So, why did they send a weaker terminator to protect John Connor from a much more powerful terminator? Were T-1000s and T-Xs too difficult to capture and reprogram? By the way, the T-800s they sent to protect John Connor were originally evil and were supposed to kill John Connor, but then the resistance captured them a reprogramed them to protect John.