Question

What is not a reason McClellan became so hesitant while marching up the peninsula?

Answer 1

Here are some of the possible reasons. Any reason not included here are not correct.
McClellan's army had landed at Fort Monroe and move up to Virginia. Winfield also retired leaving McCellan as the chief general. When Longstreet marched down during the attack on May 31, it got off on a bad start.

Answer 2

He was awaiting orders from Washington