Why did marx believe that social change was necessary in scoiety?

Karl Marx believed it. The opening two paragraphs in "The Communist Manifesto" state:

"The history of all hitherto existing society is the history of class struggles.

"Freeman and slave, patrician and plebeian, lord and serf, guildmaster and journeyman, in a Word, oppressor and oppressed, stood in constant opposition to one another, carried on an uninterrupted, now hidden, now open fight, a fight that each time ended either in revolutionary reconstitution of society at large, or in the common ruin of the struggling classes. "