Because his evaluation of the players is based on more than just 90 minutes on a Saturday, whereas that is all that the fans have to go on. So it will naturally take him a little longer to come round to the changes. Also, he's getting to know the players still.
Basic example;
Ade has a **** game, bangs them in all week in training and looks good in the system, 'definitely a player in there, maybe he'll hit it this week'. Ade has another **** game. Bangs them in again all week 'OK, he did infact look ****, but he really is looking good'. Ade has another **** game 'OK, straw that broke the camels back, it doesn't matter how he performs in training, he's not doing it on match days, can't play him anymore'.
Among about a million other variables that that fans can never understand as they aren't at training every day, I think that is what it boils down to.
Hopefully he kicks on with these core players now though.