The IRS says that form 4684 is available. why does turbotax says that it is not?

There is a little or a lot of work that needs to be done to add a tax form to a tax software system (this is true of any vendor).

First, the taxing agency has to finalize the form, then the software vendor has to make its own electronic and printed copy of the form, then the vendor has to go though a cycle with the taxing agency to get the electronic and printed versions approved, and at the same time, the software vendor has to do all the new coding for changes on the form (if any), not to mention all the regression testing that the vendor has to do to make sure that any changes on the form don't break something else. Furthermore, there has to be testing for the XML format of your tax return's forms (which is what gets e-filed, not the printed copies), to make sure that the tax software vendor and the taxing agency do the proper "hand shaking" and transmission of your tax data.

It is a non-trivial task to correctly add a form to tax software. 

As for the delays, we are not in that department, but it can happen that the taxing authority might make updates after the first of the year or that regression testing uncovers a problem that needs to be addressed. Better later and correct than sooner and not correct.