1) Night capping
2) People defending the keeps the rest of the day
3) Use of the emperor when the attackers managed to make some progress at a keep
It's now almost impossible to capture a keep if it is defended by a few people. The defenders just have to deploy a few siege weapons and wait for the cavalry to finish the job. The map will flip only at night, or will not flip at all in some campaigns, leading people to stop playing in Cyrodiil.