The Old Testament is important but it is less important than the New Testament, because the coming of Jesus represents a new covenant and a new relationship to God that supercedes the Jewish religious law and that is for all people. It's still worth studying and reading - it's a part of the religion, and it contains much virtue and wisdom - but from a theological perspective, it is much less important than the New Testament.

I think the other factor is that, if you're relatively liberal from a doctrinal perspective, the Old Testament is a little more difficult to deal with than the New Testament, because there are some issues that are more visible and difficult to deal with in the Old Testament - there's a lot of things that are not in tune with contemporary moral sensibilities and there are a lot of things that are impossible to take literally and require allegorical interpretation, and so it is much more straightforward to deal primarily with the New Testament. But that's a secondary reason.