To make sure that the chuckleheads in the state courts knew that "This MEANS YOU." Seriously, discussions of the 14th Amendment are important, and require quite a bit more bandwidth than I will be able to supply here. Believe me when I say that I sure could find a way to make the BOR apply to the states without the 14th Amendment (Supremacy Clause and various other parts of the Constitution). After consideration, I just don't believe that to be the intent of the founders. Subsequent to the 14th Amendment, no doubt in my mind whatsoever. I only wish that judges would act that way. They do not, and we now have a mess called "partial incorporation" on our hands. I do note the difference between a) the plain language of the document b) the intent of the founders (or in the case of amendments the Senate) and c) how judges actually rule in the case law.
If that was their aim, then the most sensible thing to do would be to say, "The Bill of Rights now applies against the states equally as against the federal government". By singling out that one provision in the BOR (the due process clause), they do the opposite of making that clear. Instead, they create the strong impression that only that provision is to be applied against the states. "Expressio unius exclusio alterius" and all that.
Believe me when I say that I sure could find a way to make the BOR apply to the states without the 14th Amendment (Supremacy Clause and various other parts of the Constitution).
There's been quite a lot of misunderstanding as to the supremacy clause. It has no effect on the applicability of any of the other provisions of the Constitution. Those provisions that would not apply to the states without the supremacy clause, would also not apply to the states with the supremacy clause. Those provisions that would apply to the states with the supremacy clause, would also apply to the states without the supremacy clause.
The difference between having a supremacy clause and not having one can be readily seen by looking at the situation that existed under the Articles of Confederation. Those articles contained plenty of provisions that applied to the states, but it had no supremacy clause. As a result, the provisions did not automatically trump state laws to the contrary. Instead, they merely created an "obligation" for the states to follow them. Under the new Constitution, provisions that applied to the states automatically nullified state laws that conflicted with them. That's the only effect that the supremacy clause has.
b) the intent of the founders (or in the case of amendments the Senate)
One final note, in the case of the 14th amendment, it's not just the Senate that's the equivalent of the Founders, but also the House of Representatives, and probably most importantly, the state legislatures that were charged with ratifying it.