Because the ABR-File-Format is closed source. You must pay Adobe to get their code if you want to implement it into your software. The issue here is, that Krita is open source, and because of that Adobe won’t sell the code, because Krita has to publish that according to its license.
So, as far as no one recreates the brush engine from Photoshop via reverse-engineering from scratch, you have to rely on the tedious task to manually recreate the settings.
One problem I see with rebuilding the brush engine is that the moment Adobe changes something in the ABR brush code, all the work starts again, and you have to reverse engineer that change and incorporate it into the brush engine.
If you need support in recreating the behavior of some brushes, you can ask here in the forum, there are most of the time user who can help with hints for settings or even complete brushes and bundles with lots of brushes, as @kaichi1342 mentioned before.
Maybe you take a look at the brush bundle made by @Odzuki? You can download it from here (and please use this link, because it links to the current version of this bundle): EDIT, new link:
But I recommend you to read the topic belonging to this bundle too, (nearly?) every brush is presented there with a good description, sample pictures and sometimes even a short video. The topic can be found here (but Odzuki’s links no longer work and so I took over hosting them):
Michelist