The best animal for sacrifice is a camel, then a cow, then a sheep, then a share of a camel or cow, because the Prophet (peace be upon him) said concerning Jumu`ah (Friday) Prayer,
"...Whoever goes for the prayer (in the first hour i.e. early), it is as if he had sacrificed a camel (in Allah's cause); and whoever goes in the second hour it is as if he had sacrificed a cow; and whoever goes in the third hour, then it is as if he had sacrificed a horned ram; and if one goes in the fourth hour, then it is as if he had sacrificed a hen; and whoever goes in the fifth hour then it is as if he had offered an egg..."
(Part No. 11; Page No. 399) The point here is that there is an order of preference between camels, cows and sheep in drawing close to Allah through the sacrifice. Undoubtedly sacrifice is one of the greatest acts of worship through which we may draw closer to Allah. A camel is more valuable and more useful. This is the view of the three Imams
Imam Malik said: The best is a young sheep, then a cow, then a camel, because the Prophet (peace be upon him) sacrificed two rams, and he (peace be upon him) did not do anything but that which was best.
The answer to this is:
The Prophet (peace be upon him) sometimes chose the option that was not the best option out of kindness to his Ummah (nation), because they follow his example. He (peace be upon him) did not want to make things difficult for them. He stated that the best was a camel, then a cow, then a sheep, as stated above. Allah knows best.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.