Answer:
Read the explanation section.
Step-by-step explanation:
We know, sand can't contain water because of it's nature. But mud can contain water. For preservation of an organisms, sometimes water is needed because many microbes needs water which helps to preserve the organisms. Along with this, mud has the nature of holding things for long time inside of it but sands doesn't have this.
As sand can't store water so that microbes don't do their work properly and that's why preservation get hampered.
But in mud, this problem isn't present.