1) Storage at Cushing, OK, which is the delivery point for WTI, is upwards of 33 MM bbls. The industry only needs 19-20 MM bbls, so there's relative oversupply, thus depressing WTI price more than other grades (and also creating/maintaining the huge month/month contango).
2) Brent supplies are growing tighter over time, and will continue to do so as the North Sea fields become more and more exhausted. This mitigates the general downtrend in crude prices with regard to Brent, making it relatively stronger (or relatively less weak, if you like).
I am surpsrised there are no new finds for Brent oil.