Here's a couple of reasons:
You will need good blocking
You will need good situational awareness
You will find yourself being shot to pieces alot
You will find yourself overwhelmed by cavalry in the open (on populated servers)
Not saying that being a 2hander is any harder than shielder, but the game is different when you are a 2hander, and you will need to play it differently.
If you find yourself having trouble against 2handers, remember your advantage:
You have a temporary autoblock (Your shield)
The shield allows you to do things that are suicidal to 2handers:
You can approach groups of archers, flank, and if lucky or against unskilled opponents, start chopping them to pieces.
You are much harder to get rid off. Standing on a hill and just looking on the enemy makes you a threat, and they might NEED to respond. 2handers have to charge right away.
You can fight groups of enemies much easier, and with practice and against unskilled opponents, you can defeat alot of enemies.
You can protect your allies from multiple enemies or ranged attacks, giving them the opportunity to counterattack while you keep your shield up.
Just trying to help you out.