Start/Complete Full-Round Action
The “start full-round action” standard action lets you start undertaking a full-round action, which you can complete in the following round by using another standard action. You can't use this action to start or complete a full attack, charge, run, or withdraw.
What if you could start a full attack and finish it next turn?
How would this work?
Split your attacks up and if the enemy moves away before you're next turn your sitting next turn with a single move?
It would be a mess, I think. How many of your attacks would you be able to use during the first standard action, and how many during the second (if you have an odd number of attacks).
Plus, if you have more than 2 attacks, this would let you get more attacks in total over the course of 2 rounds and still move just as much (by using move+standard, then standard+move) than you would by using a move+standard action each of those rounds.
Also, the player would likely start with his highest-attack-bonus attacks, so potentially losing the less efficient attacks in case the enemy moved away wouldn't really be a loss. He still got multiple high-attack-bonus attacks in, which he would also have done by using standard action attacks in each of those two rounds.
The limitation is there for a reason :)