Organisms are well preserved in mud because mud has high water holding capacity. It retains water and does not allow oxygen flow which can destroy the remains of organisms. There are some microbes that preserved whose growth are enhanced in water related environment like mud.
But sand cannot retain or hold water. It drains it off and all oxygen bearing water to flow which enhance decay compared to mud. It can't preserve organisms because of the above reasons.
Organisms are more likely to be well preserved in mud than in sand because sand allows oxygen that has water(oxygen-bearing water) to flow through it. This type of oxygen speeds up decay. Mud does not have this oxygen so it prevents the flow of water, and keeping oxygen away means it slows decay of the buried organisms.
Hope this helps! :D
You might want to write more and/or change it a little because my teacher only gave me half points for this. :(
