When Tesla developed their connector (now North American Charging Standard, NACS), the Combined Charging System did not exist. CCS, however, still wasn't developed by the government. The Society of Automotive Engineers issued the charging specifications for the United States as standard J1772; however, this isn't adopted as a national standard. J1772 is where the CCS implementation lies in the US.
When Tesla needed fast Level 2 charging (faster than 6.6kW), they had to develop their own standard. When Tesla needed DC fast charging faster than 50kW and less bulky than the CHAdeMO connector, they had to develop their own. They developed their own connector such that it could do *both* things, and do them well, and be able to extend it in the future with substantial backwards compatibility. Today, a single connector can handle 1.4kW (120@12A) all the way up to 1MW (NACS current max). J1772/CCS1 is a mess, and it's not compatible with the rest of the world's CCS connectors. Tesla was able to extend *their* connector to support CCS by upgrading the hardware on the charge ports and the supercharging stations to add the communications protocol.
And through all of that, they built out a charging network that is larger than the largest CCS network (Electrify America). They have a substantial majority of cars on the road using their network. They've developed a mechanism that allows CCS users to use their stations without having to bring their own adapter. There really isn't incentive to do anything else unless the government wants to declare a specific connector as the standard, which they haven't.