The way MahAbhArata was fought was not the standard practice even during then. Had Arjuna used celestial weapons (DivyAstra), it would have gained a quick victory, as it happened in VirAta Yudha. Then why they fixed upon to use limited weapons?
The premises of MahAbhArata war was the conflict within the family, predominantly between step-cousins. The war was fought in Kurukshetra to avoid harming other population and reluctant kingdoms (who din't want to join). There was no intention to invade the territories. Rules of Engagement were laid before the war to maintain the ethical conduct, because this was a Dharma Yudha, i.e. the war fought to maintain the righteousness. Look at below rule:
Two warriors may "duel", or engage in prolonged personal combat, only
if they carry the same weapons and they are on the same type of mount
(on foot, on a horse, on an elephant, or in a chariot).
Which means, either Arjuna can't fight or he must break the rule for using DivyAstra. Both were not acceptable. Moreover, Drona and AshwathAmA also possessed such weapons in their arsenal. Bhima's grandson Barbarika wasn't allowed to fight for the same reasons; i.e. using special weapons.
The celestial weapons might not have been allowed for the same reason why the atomic warfare is not happening today. No one wins such wars, but humanity surely loses.
Interestingly, by the end of the Kurukshetra war, almost all the rules were broken in 1 or the other form. Because they were rules (or promises), they were destined to be broken!