Charlton fans had gotten tired of Robinson and his excuses. They had gotten tired of his tactics and playing style.
As someone else mentioned above, simply replace Charlton with Oxford, and Robinson with Pep, and it's exactly the same situation.
Quite simply, under Pep we weren't improving; we were treading water (at best). On 1st Oct, we were in 13th place, 4 pts off the playoffs. On 1st Nov, we were in 10th, and 2 pts away. 1st Dec, 9th place and 8 pts. 1st Jan, 10th place and 6 pts, 22nd Jan, 10th and 8 pts (and having played one game more).
And after the 1st Nov when we WERE in touching distance of the playoffs, Pep managed to lose at home to 21st placed Northampton, get a goalless draw at Rochdale who were only out of the relegation zone on goal difference, lose at home to bottom-of-the-table Bury, and ship 11 (Eleven) goals in two home games against the top-of-the-table teams.
So between 1st Nov and when he was sacked, Pep had taken the team from 2 pts away from a playoff spot to, effectively, 11 pts. But, yeah, of course he was going to turn it around.....