Really? You know NOBODY really needs 1TB of storage? I have a feeling the people who work with raw images and videos and will be using the 2019 version of full photoshops, taking their iPad on the road to clients, etc. are going to disagree with you.
Storage CAN dictate RAM. A 1 TB drive needs more RAM to aid in it. In another thread on these forums somebody has the numbers, and a 1 TB drive required 0.5 MB of RAM more than the lesser storage options. 4.5 GB and 5GB of memory don't really exist all that much, so Apple had to go up to 6 GB. On the flip side, if you are actually after the most RAM, a smaller storage solution requires a smaller amount of RAM for the cache. So if all the other models do have 4GB then the ones with less RAM would have more access to it.
That aside...did any of the reviewers really even tax the iPad to show even a 4GB would have been taxed? By this I mean, short of some hefty hard-hitting programs that are not even out yet, or maybe hard hitting games (which won't be developed JUST for the 1 TB 6GB iPad anyway), many people are not going to be seeing the difference in real life, and reviewers didn't have access to programs written to take advantage of 6 GB either.