Answer:
We know, sand can't contain water because of its nature. But mud can contain water. For preservation of an organisms, sometimes water is needed because many microbes need water which helps to preserve the organisms. Along with this, mud has the nature of holding things for a long time inside of it but sands do not have this.
As sand can't store water so that microbes don't do their work properly and that's why preservation gets hampered.
But in mud, this problem isn't present.
(IN YOUR OWN WORDS OF COURSE.)
