I've done my fair share of software upgrades on complex systems. It's very apparent that Tesla only recently made the decision to develop the range limiting software patch. Tesla software engineers must determine the requirements for the software, design it, code it, test it and regression test it.
This software may only be about 500-1000 source lines of code, but even this still may require 30-60 days from start to finish to complete, depending on how it's implemented. As a customer, it's in your best interest that Tesla doesn't rush it - we don't want software bugs discovered post-delivery.