Hitler's aides did what they could to comply with his stated wishes to be cremated to deprive the Soviets of a body, but they could only do so much in the circumstances. A large amount of fuel was requisitioned and used to cover the remains for burning, but while there is debate on precisely how much the Soviets actually recovered (See Joachinsthaler's "The Last Days of Hitler") they certainly recovered something, enough at the very least to make a dental identification, which is to be expected as total cremation was assuredly impossible in the circumstances. Even proper crematories, which are purpose-built for the task, don't actually turn a body entirely to ashes. What a family receives in an urn after the body is professionally cremated, there are still bone fragments and other "pieces" which are not entirely burnt to ash which go through a process called "cremulation" - crematories will usually have a machine that does this - to turn those cremains into a fairly uniform pile of ashes.

So put simply, there was neither the time nor the facilities to entirely render Hitler's body into a literal pile of ashes. They were burning the body in the open air, not a proper crematorium, and they lacked by sort of machine for cremulation.

"The Last Days of Hitler" by Anton Joachimsthaler

Mates, Lewis H.. Encyclopedia of Cremation, edited by Douglas J. Davies, Routledge, 2010.