Because ...
... contrary to popular belief, you can get Covid-19 more than once. And the effects can be less, the same, or worse the second (third ...) time around. Getting the jabs on schedule minimize the effects of a second (third ...) case.
Why the rules are different in different countries are as varied as any other laws in the various countries ... mostly it's down to the fickleness of how elected officials perceive their electorate.