Firstly there is no place in the Quran that recommend this form of burial. Secondly, the Holy Prophet (SAW) nor his Sahaba practiced this form of burial so one cannot say this is Sunna. Thirdly , those observers watching a dead body burn actually relates it to hell fire and nobody wants to think that his loved one is going through that kind of "punishment" even though his "trial" had not even come. Fourthly, the ashes of a cremated person is generally not buried but tossed somewhere (usually at sea) and that is unhealthy. Fifthly, one is deprived of visiting his loved one at a marked grave and also to pray for him/her when he does visit.