I can't say for certain why they've decided to nerf the tax percentage, and maybe the reasons that people have already listed in this thread are the correct answers, but one thing to consider is that it appears they've also made some minor changes to the passive food income of settlements.
The "Land around settlements" bonus that all settlements used to have is back and has been rebranded as "Inside Production". Consequently, they've also reduced the food given per level of Orchards, but the sum total of the bonuses from Inside Production and Orchards in 1.6.1 is still higher than Orchards was alone in 1.6.0. Since the baseline food income that settlements have access to has increased across the board, the average prosperity of settlements should also rise, thus resulting in higher tax income.
Remember, when the game first released, the tax percentage from prosperity was only 20%. It was later buffed to 25%, then to 36%, and then nerfed to the present 30%. So even with the nerf, the tax percentage is still a full 10% higher than it was at launch. Since launch though, the average prosperity of settlements has fallen quite a bit (which would reduce tax income) and loyalty and security values tend to be lower (which also reduces tax income), which is why they probably felt a bump to tax percentages was necessary to balance things out. Now that settlements' passive food income has been buffed a bit, they might feel a minor decrease to tax percentages is warranted to balance out the higher expected prosperity.
It's probably best not to evaluate any changes they make in a vacuum since the game's mechanics are so interconnected, and balance tweaks to one mechanic are bound to also affect adjacent mechanics. They have the data collection tools to keep track of the broad effects any changes they may make will have.