tl;dr to original poster:
They are not stronger. I am sorry to say, but they are just plain better players than you are. Try and take a twohander yourself and see how you deal with it.
I really don't care much for these balance discussion. Shame shit over and over.
I started out as a shielder because I sucked at blocking. Then I learned blocking and started as a two-hander with decent results.
Then I played cav get great results with my morningstar and horse (talking about a once op class) switching to twohander up close.
Then I went polearm with great results and now I am one-hander with shield once again.
What did I learn?
1) Two-handers and polearm players are not inherently faster as a class than one-handers. The players tend to be more experienced and just plain better players. Give Phyrex, Siiem, Jormglorm, vMega and what not a shield, and they would kick you ass too. There are just some people currently playing twohanders or polearms that are extremely skilled. This has nothing to do with class.
2) As a cav in a pub server you get great K/D. People are to unaware to deal with them.
3) As a shielder I am not rarely in the top 5. Other great shielders such as Vicious, Kingrimm, Olwen what not are often there. I believe a lot of two-handers fear those players.
Why did I change to shielder? (with a pokey side arm)
Because in team play they are invaluable. Plain and simple. With enough shielders you shocktroops and archers have protection while they do the killing.
Just wait for strat... Shields are needed in organised team play, where individual skills matters less (because the overall skill is higher).