The long and the short of it is because most keyboards don't have dedicated keys for such characters. Ligatures are here, today, and everyone can make use of them. Rich unicode characters require input method changes that are custom to every platform (and likely need to be customised for each international keyboard layout as well).

This is not a hypothetical, btw. One of the most persistent friction points for C++ adoption in Europe was the lack of a ~ (tilde) key, forcing folks to ALT-1-2-6 every time they wanted to implement a destructor...