Because escaping the annoyances of the "modern web" may require escaping from the "modern browser". The changes users want to "modern browsers" will never be made. The vendors of these programs do not answer to users. They answer to web developers and advertisers. These are large, complex, insecure programs usually controlled by organisations that seek to profit from online advertising. The advertising focus leads to complex web pages. Not the type of simpler pages that some users want. (NB. Not "most", but "some".) Gemini, because of its limitations, allows users to retrieve resources without the need for one of these "modern browser" programs. If a new Content-Type was added to HTTP, what is the likelihood that other parties outside the "modern web browser" cabal would write small, simple, alternative browsers, e.g., aimed only at this Content-Type. Look at the market share, i.e., available selection, of web browsers. It is not diverse.
Whereas, writing a Gemini client is dead simple. A Gemini browser cabal where users have only a few choices and they are each controlled by corporations is unlikely.
Asking web developers to "please make simpler web pages, thanks", when the "modern browser" allows for complex pages and integration of advertising is not a succesful course of action. Most of these web developers answer to advertisers or to employers who answer to advertisers. They are not going to ditch the user annoyances, they are going to seek profits. Gemini seems to address this problem by making advertising difficult. Without the "modern browser", the possibilities for advertising are limited.
Similarly, asking web users to "please use a text-only browser", e.g., Links, when so many web pages try to use "modern" browser features that enable complex web pages is probably not a successful strategy for many users either. As a long-time Links user, that strategy has worked for me, though.
The only complaint I have about Gemini is the absolute requirement for SNI. Not every IP addresss will necessarily be hosting multiple Gemini sites. Under the current protocol, even addresses hosting only a single site must require SNI. That makes no sense. It serves no purpose. It should be optional not mandatory.