Why did Bhishma and Drona fight on the side of the Kauravas?

From: Purushottam Kumar

Please clarify Bhisma & Drona’s position?

Karna’s loyalty towards Duryodhana is condemned whereas Bhisma and Drona’s loyalty towards Duryodhana and Dhritarashtrais justified.  When Draupadi was being shamed in the assembly hall then neither Bhisma nor Drona rose to protect her honour. Being the eldest member & guru of the Kuru dynasty respectively was it not their responsibility to stop this inhuman act?

Also why Bhisma and Drona did not refuse to fight along with the army of Duryodhana. Is it not true that Duryodhana’s confidence was based on the fact that he had the greatest warriors like Bhisma and Drona with him who could never be defeated?

We see that in Ramayan when Vibhishan’s all attempt to bring Ravana on the right path fails then he leaves Ravana, leaves Lanka and takes side of Lord Rama and joins Lord Rama’s army. Why Bhisma and Drona did not do the same thing.

Request you to please clarify this.