Apache is heavy when compared to lighttpd, and Pi-hole's initial focus when it started was to perform well on very restricted hardware.
The real question here is not why Pi-hole should prefer one webserver over another, but whether Pi-hole should restrict your choice of web servers at all.
And Pi-hole doesn't do so, even in its current version:
You can opt to NOT install its lighttpd webserver during installation, but only the web UI components (or not even those).
You just have to figure how to integrate those with the webserver of your choice, which shouldn't pose a big issue if you are familiar with the webserver you prefer over lighttpd.
That said, the upcoming Pi-holev6 release will do away with lighttpd as well as some other web-related package dependencies, which should certainly make it easier to deploy Pi-hole's UI.