In the United States at this time Liberalism is not only the dominant but even the sole intellectual tradition. For it is the plain fact that nowadays there are no conservative or reactionary ideas in general circulation.
Trilling thought that there were impulses to conservatism, even strong ones, in the population, but that these weren't articulated and so didn't challenge the liberal stranglehold on political ideas.
Why point this out? First, it helps to explain why Western society has veered so radically down a liberal path: there was no well articulated challenge to the liberal orthodoxy to hold things back.
It's also important that we remind people that the Western political establishment is and has been a liberal and not a conservative one. If things have gone wrong it is because liberals were at the helm. If conservatives have been at fault it's for not articulating a principled opposition to liberalism of both the classical (right) and social democratic (left) varieties.