what happened to all the fossils?!

In "Lost in a Million-Year Gap, Solid Clues to Human Origins" William Kimbel tells us about fossils from 2.6 million years ago when the Homo Habilis roamed the earth. Apparently hominid fossils are hard to find from that period, he said,  “It’s not that sites containing rocks this age are particularly rare, or that the time period in eastern Africa has not been searched by several groups... The problem is that the fossil yield has thus far been low or poorly preserved, compared to the time periods on either side of this interval.” But why is that? He does not explain any further on the subject, but it is something that baffles me. Was it something in the soil that couldn't preserve the fossils? Or was it pure negligence from the paleoanthropologists? How can there be plenty of fossils from before and after that period but that one period in time there is not enough? Although this was just a short paragraph in the New York Times article, it struck me because it left me wondering. so I tried to answer these questions with my own research. I googled "dark Age," which was a bad decision because it seems like every continent or country had a dark age at some point. next i tried "dark age Homo Habilis" and i got this New York times article and a whole lot about how the habilis species were known as "handy men" and made tools, which i already knew. But i could not find anything on the lack of fossils from that period. So, overall i found the article frustrating to read because i read it waiting for it to mention more about this fossil dark age and it never did. Such a shame