(1) A lot of people dive headfirst into applying what they believe to be the beliefs of the Founders without first considering whether we should even be following those beliefs in the first place.
(2) There is no consensus about the beliefs of the Founders, and current views about those beliefs are often severely distorted by bias. For example, the Founders are viewed as everything from a bunch of bible-pounding holy rolling fundies to a bunch of godless blasphemous atheists. In one of the worst examples of originalism, Judge John E. Jones III said in a Dickinson College commencement speech that his decision in the Kitzmiller v. Dover intelligent design case was influenced by his notion that the Founders believed that organized religions are not "true" religions.
(3) The Founders were not monolithic in their beliefs.
(4) After a span of two centuries, it is difficult to discern the beliefs of the Founders, particularly the lesser-known Founders.
(5) The Founders would not have been happy living under all of our principles, so why should we be happy living under all of theirs?
(6) The Founders made mistakes and omissions. For example, the Founders failed to write the Constitution in a way that would have prevented the Civil War. The courts had to add an imaginary "dormant" comment clause -- i.e., a general prohibition on state interference with interstate commerce -- because such a clause was not contained in the Constitution.
(7) Many issues today were not even on the radar screens of the Founders, e.g., environmental problems and freedom of expression on the Internet.
(8) It is argued that originalist interpretations can be overridden by amendment of the Constitution, but amending the Constitution is extremely difficult. There has been no significant amendment of the Constitution since the 1971 amendment which lowered the voting age to 18.
I am certainly interested in knowing the opinions of the Founders, but IMO we should not be bound by those opinions and those opinions should be taken with a grain of salt. It has been said that originalism can be a form of judicial activism, and I agree. IMO the overemphasis on orginalism is going to lead to a backlash against the Founders.