Presumably because they're "pop-up" ie quick implementations and it's quicker to put lollipops or whatever on existing tarmac than to tarmac over grass.
That's in addition to the value of the grass itself, in soaking up heavy rain, habitat for insects, less solar absorption on a hot day, and all the other benefits of greenery. And that they're redistributing space from motor traffic to non-motor traffic rather than pedestrians to cyclists.
I'm impressed if they actually will be 2.2m width.