It was just a matter of taste on the C# langauge designer team. I am willing to bet it was for comonality with other C family languages. It does however lead to some bad programming practices such as a private field and its associalted property differing only in the case of the first letter.
EDIT:
Why might this be cosidered bad.
class SomeClass
{
private int someField;
public int SomeField
{
get { return SomeField; }
// now we have recursion where its not wanted and its
// difficult for the eye to pick out and results in a
// StackOverflowException.
}
}
Prefixing private fields with an _ or an m might make it easier to spot. Its not a huge biggie and personally I sill do exactly what I have just said is bad (so sue me!).