You have to account for the fact that several states have equivalent provisions in their constitutions, some of them explicitly protecting individual rights, and other states have added such provisions shortly after.

That is, the right was broadly recognized as an individual one and not just a state/federal interaction issue early on. The federal constitution didn't consider that case for the same reason why it didn't consider them for all the other amendments - it was simply out of scope there then. But given the historical recognition of the right at the time 14A was enacted, it makes sense to incorporate it.