People underestimate the "brand value" of legacy Bitcoin. People will always herd to what is proven and accomplishes their needs.

It is very difficult to make people change their habits and, in the case of legacy bitcoin, it also has the best minds in the field working on the code, so there is a positive expectancy in terms of future features, not just security.

And when most forks are mired in controversial motives and led by sketchy characters, that further helps strengthen the original bitcoin.