I kind of get why the rewrite needs to happen. But I also see that this rewrite is an unfortunate waste of time and resources.
The reason is the same why a core Emacs package wouldn't rely on a non-core package, no matter how useful or awesome that package might be.
For example, we cannot have a core package, say vc.el rely on hydra.el even though it's in GNU Elpa. It would have been awesome to see all the vc actions in the hydra interface, but that cannot happen until hydra.el is part of the core.
With Org/ox-html, it's the same thing. Ox-html is part of Emacs core. So it cannot rely on htmlize.el. The author doesn't want to assign the copyright of the package to FSF. So it cannot be a part of Emacs core (or even GNU Elpa).
[Captain hindsight: htmlize.el shouldn't have been used in ox-html in the first place without the author having assigned the copyright to FSF.]