Not an actual historian, but as I recall the Norman invasion fleet had been delayed for weeks due to bad winds (Technological limitations meant that they had to wait for a northerly wind to sail for England). Harold couldn't just wait endlessly for William to show up - Much of his army was comprised of the Fyrd, a levy of non-professional troops who he could legally only keep in the field for a limited time before they would be allowed to return to their homes, farms etc.

If he'd waited, he would potentially ended up with 2 hostile armies, and no troops to fight them with.