>military of Donetsk's Peoples Republic would grant free access
Currently inspectors are on the site gathering people's belongings.
But it is still an active war zone, some 20km from the front line. (very heavy fighting at Debaltseve 10km (or so) from nearest plane debris, 25km from Hrabove)
Untill recently the area has had limited access and the rest considered dangerous.
Other than that, yes, really weird why some of the wreck is still there (only stolen parts moved away).
More... my 100% amateur estimation of the situation on crash zone (under current "ceasefire"):
Rebel motorized howizers and Grad lauchers move around the area to daily shell Ukrainian positions, there might be ammunition flying above the crash site even.
When Ukrainian postitions are attacked, Ukraine artilery radars find out the trajectory of the hostile fire and they most likely fire back to the previous estimated location of the howizer or grad launcher.
Howizers reach 20...55 km depending on ammunition, grad rockets roughly similarly. (I assume rebels do not have the latest far reaching ammunition, though)
So, I could be totally wrong, but as the ceasefire is very fragile, it is insanely dangerous to be on the crash site.
UPDATE: In the following image the yellow line is the front line, it even crosses the MH17 debris zone. Also one can see how close the shells hit.
https://pbs.twimg.com/media/B0uZ5TjIUAAc4-k.png