Why is the check engine light still on after o2 sensor has been replaced

You need to clear the code once a repair is done. If you replaced the oxygen sensor just because the light is on or because you had a code for an oxygen sensor, doesn't mean a new sensor will correct the problem. The C.E.L. can on for hundreds of reasons. Even if you have a code for a specific component doesn't mean that component is at fault. The code isn't the answer to the repair, the code gives the technician the direction he needs to go with his diagnostic procedure to determine the correct repair.