The decisions haven't really been explained by the standards of explanation you might expect for such a move, at least not everywhere.
In Wales the official explanation is: Welsh ministers said a 20mph (32km/h) limit would reduce deaths and noise and encourage people to walk or cycle [1]. Drakeford elaborated by saying "It's going to take you a minute longer to make your journey, and we will save 10 people's lives in Wales every year as a result of that one minute contribution - it doesn't seem an unfair bargain".
This explanation is nonsensical:
1. It could just as well apply to a limit of 5mph, or 10mph, or banning cars completely. It explains the desire to lower the limit but not the choice of 20mph vs 30mph.
2. The claim is that it adds a "minute longer" to make your journey, a clearly confabulated number. Not all journeys are the same and lowering the speed limit nationally will clearly be a scaling factor to your journey time, not a constant minute regardless of distance.
3. The justification is a 100% subjective feeling that "it doesn't seem unfair". This isn't an explanation, just an emotion.
When politicians serve up gibberish explanations, people tend to split down the middle. One half will assume that politicians are just being really stupid. Others will say that nobody can be that stupid, so there must be some alternative agenda at work.
Certainly this looks to people like "post-evidence politics" albeit not in the way you probably imagine.
It doesn't help that these people are socialists who have a long history of presenting one explanation for their policies whilst actually having another. When they're in friendly territory they tend to talk about how much they strongly desired things like enforced equality of outcomes, degrowth and reducing CO2 emissions, but they never talk about pedestrian safety. So when they impose policies that will clearly have the effect of reducing growth and car usage, but justify it with some new cause they hardly talked about before, it doubles up suspicion again.
BTW the 20mph limit didn't appear in any manifesto. More people have now signed a petition against it than voted Labour in the first place. So they can't claim these policies are popular or democratic.
[1] https://www.bbc.com/news/uk-wales-66774379