The simple reason for this is ... 2h weapons have more damage.
Their swings aren't faster but you can speed them up by turning into them, which lowers damage. And since 2h have more damage, which is even more exagerated by all the boni you can stack, they can be speed up much more, to the point that an imperfectly done counterattack will always fail. I don't have problems counterattacking with 1h normally, but getting damage is kind of hard that way.
Also, this speeding up is best done with movement, since that adds damage to compensate, and shield slows you down.
But, this is an important balance consideration - if 2h has to block (this includes feints, spinthrusts, sudden "lunges", rythm changes, as well as fighting against multiple enemies), and have a high risk to be killed by ranged weapons, he must have much higher killing potential. There are some things I don't really like at the system, but just saying, swinging my staff without pause is my impenetrable steel wall.
Good techniques for shield users seem to be to wait behind your shield until a clear opportunity arises and pressure them when you have that chance. Also, fight tactically, engage throwers and pikemen when you can, these could kill the twohander that will kill the entire enemy team, and you will have little problem with them, just let one or two enemy twohanders whack at your shield for 30 seconds and they won't be able to kill anyone ... you may not see that on the scoresheet, but you might just have won the round.