When you see that a .com is already a developed business site, do you think that you must be able to upgrade to the .com one day in the future and then use another extension temporarily and wait for that day? In this case, if companies choose to use other extensions, it means that .com is not a must to them because they know they may never own .com versions in the future.
Of course, if .com is available for sale, start-ups will try to buy it when they can afford it because .com is still the most used extension.