The Constitution is an evolving document. So applying originalist, 18th-century thinking to certain parts of it, but not to other parts of it, leads to inconsistency and incoherence.
Basically, you can't say that a certain part of the Constitution should be interpreted as it was in the 18th century, but a different part should be interpreted by today's standards.
Same thing with the Bible; Conservatives abuse the notions there too.
The Bible and the Constitution are the two documents Conservatives cite the most, but have never really fully read either.