It's quite a increase in level.........
A lot of folks might really struggle to reach that B1 level.
I'm just guessing, but I would think that the level of English competency for speaking and listening would be much better after having lived in UK for a few years allowing a much better chance to progress.
The government had been heavily criticised for granting settlement and British Citizenship to people who could hardly speak or understand any English. Hence the uplift in requirement.
The idea is to improve integration by improving English Language skills