Top companies change every decade in a capitalistic society thanks to free market, new innovations, products and services.
Companies that dominated the list in one decade may become either obsolete (because did not re-invent the business model or adapt to emerging trends) and went bankrupt or become a smaller player.
For example AT&T was in the top of the list thanks to the revolution in the Telecommunications industry and deployments of telephone lines, fax machines and networks across the U.S and the globe. They eventually got broken up into smaller parts in the 90’s and become a much smaller player.
Exxon was a big player in the 1990s and 2010 due to the boom in the oil prices and strong growth in the Global economy. But now due to over supply of Oil (additional supply from countries like Iraq contributing to overall supply), shift in the market from oil to Electric in light vehicle production (Tesla slowly taking over the auto market) contributed to decline in Exxon’s market capital.