The top level domains started that way,

.COM - commercial (companies)
.ORG - organizations
.NET - network providers
.MIL - military
.GOV - government
.EDU - education

This worked out fine when there was one company administering the whole thing. However, over time the lines have blurred to the point where companies will purchase the domain under a number of top-level domains. The .GOV, .EDU, and .MIL are still unavailable to the general public.

Here is an interesting writeup from ICANN

In the 1980s, seven gTLDs (.com, .edu, .gov, .int, .mil, .net, and .org) were created. Domain names may be registered in three of these (.com, .net, and .org) without restriction; the other four have limited purposes.

Over the next twelve years, various discussions occurred concerning additional gTLDs, leading to the selection in November 2000 of seven new TLDs for introduction. These were introduced in 2001 and 2002. Four of the new TLDs (.biz, .info, .name, and .pro) are unsponsored. The other three new TLDs (.aero, .coop, and .museum) are sponsored.

Generally speaking, an unsponsored TLD operates under policies established by the global Internet community directly through the ICANN process, while a sponsored TLD is a specialized TLD that has a sponsor representing the narrower community that is most affected by the TLD. The sponsor thus carries out delegated policy-formulation responsibilities over many matters concerning the TLD.