We all know Vidura for his intellectual skills and as an advisior. But also why he didn't participate? Is it right to say that he was against Kurukshetra war? Is Krishna the reason for it again?
First Vidura was not considered as Kshatriya and He was insulted by Duryodhana (see below)
Lord Krsna, the Personality of Godhead, as accepted the sons of Prtha as His kinsmen, and all the kings of the world are with Lord Sri Krsna. He is present in His home with all His family members, the kings and princes of the Yadu dynasty, who have conquered an unlimited number of rulers, and He is their Lord. You are maintaining offense personified, Duryodhana, as your infallible son, but he is envious of Lord Krsna. And because you are thus maintaining a nondevotee of Krsna, you are devoid of all auspicious qualities. Relieve yourself of this ill fortune as soon as possible and do good to the whole family! While speaking thus, Vidura, whose personal character was esteemed by respectable persons, was insulted by Duryodhana, who was swollen with anger and whose lips were trembling. Duryodhana was in company with Karna, his younger brothers and his maternal uncle Sakuni. Who asked him to come here, this son of a kept mistress? He is so crooked that he spies in the interest of the enemy against those on whose support he has grown up. Toss him out of the palace immediately and leave him with only his breath. Thus being pierced by arrows through his ears and afflicted to the core of his heart, Vidura placed his bow on the door and quit his brother’s palace. He was not sorry, for he considered the acts of the external energy to be supreme. By his piety, Vidura achieved the advantages of the pious Kauravas. After leaving Hastinapura, he took shelter of many places of pilgrimages, which are the Lord’s lotus feet. With a desire to gain a high order of pious life, he traveled to holy places where thousands of transcendental forms of the Lord are situated. He began to travel alone, thinking only of Krsna, through various holy places like Ayodhya, Dvaraka and Mathura. He traveled where the grove, hill, orchard, river and lake are all pure and sinless and where the forms of the Unlimited decorate the temples. Thus he performed the pilgrim’s progress.
Source : [Srimad Bhagavatam 3.1.12-18]