When you are writing a tax program that will only be used by professional tax preparers, you can be a lot less careful on how the information is entered on the form. You can assume that the professional knows exactly what to do, why the from is used and could fill in the form correctly without software.
When you are writing a program to that non-professional will use, you have to consider a lot more options. You cannot assume that people will understand the form and how to enter the data for it. TurboTax keeps testing their program until it is so foolproof that you could put a person at the keyboard who has no knowledge of tax law, and they can answer questions and get the form to complete correctly.
The IRS scrutinizes the programming, and it has to be fool proof before it is released. That is why it takes longer.
