Well, in theory, it actually wasn't. Nitter doesn't use the official Twitter API, which you can easily block access to, but rather uses the webpage API. Blocking access to the webpage API requires blocking access to the whole webpage without user login, which no one expected would ever happen for a service which's main use is to publicly announce things.

Well, and then came the great scraping to feed the LLMs + the questionable sanity of Musk, which meant Twitter did actually block public access to the webpage.