If WTI sells a too big a premium over brent, people will import brent and not buy WTI getting prices to be closer over time.
When the opposite happens like right now, that mechanism doesnt exist because WTI from Cushing can only be sent to Chicago(people trying to take advantage of that in the UK, cant) and if there is no demand by the refiners there(and it should be the case given the recession) then the WTI just keeps being stock pilled more and more and driving the price down