hmm what about at the great naval battle of Dan-no -ura Minamoto no Yoshitsune is descivley beaten by the Taira. In otl it was Minamoto who established the legacy of the warrior castes, united them, and created his military bases with their support. So perhaps if you can have the Minamoto clan lose this naval battle, coupled with a crushing defeat, perhaps Minamoto and the warrior clans of Kanto could be beaten. If they are indeed beaten then Yoshitsune never becomes Shogun.
On top of that lets say for their revolt, the victorious Taira clan deprives these military clans of their lands o jails them so as to say as retribution. Or at the very least weakens their power.
Now I'm not sure if the Taira had plans for creating their own shogunate but if they didnt and continued to support the emperor, than I doubt the court would see value in the development of an independent class called the samurai who were mainly drawn from the warrior clans. After all if the warrior clans were beatean they were not so special and so samurai if they did develop which they would have eventually, would be relegated to the military. T
he key difference from otl being a civil government resides in the capital and the military remains separate from said government.
Another POD could be even earlier, lets say the Taira ally with the Fujiwara somehow to crush Minamoto and then the two sides fight leading to a Fujiwara triumph. Or just having the Fujiwara by some miracle defeat the Minamoto-Taira allaince, perhaps with aid from the Hojo? If Fujiwara clan triumphs they will try to preserve the status quo of the civil government since the Fujiwara played a very important role during the time period and their princesses were married to the emperors.