NOTE: of the 48 Cardinals from Italy, only 26 can vote
Italy has the most for several reasons (I list some reasons below):
at least 29 of the living 48 originally became Cardinal-Deacons due to their work in the Roman Curia. Plus, Secretary of State Cardinal Parolin went straight to Cardinal-Priest even though he never lead a diocese (Most of the Curia is historically made up of Italians, as they have enough priests to take of their flock and the Curia, at least compared against other nations, and are local).
others are Archbishops of major archdioceses, which have historically been granted Cardinal status.
many were also Apostolic Pro-Nuncios and/or representatives to international bodies, again a lot of these have been Italians.
at least 4 or 5 Italian Cardinals were created Cardinal after his 80th birthday, meaning he could never vote. For example: Cardinal Capovilla was created Cardinal by Pope Francis this year, when he was 98 years old.
NOW, what’s really shocking isn’t the number of Italian Cardinals, it’s the number of American Cardinals. The United States has the 2nd most, with 18 Cardinals (11 voting)
In the future, as the Roman Curia starts to be lead by more international priests/bishops (like Cardinals Burke, Pell and Muller), I suspect the number of Italian Cardinals will decrease. But even if their numbers do decrease, I would imagine they will still most likely still have the most for the foreseeable future.