^I was wondering that too.
I think it would be better with trilliums instead of maple leaves, so it's specific to Ontario.
But I believe the trillium was a more recent adoption, (60's?) whereas the triple maple leaf has been a symbol on Ontario's coat of arms for ages. Let me look into it.
EDIT:
TRILLIUM HISTORY:
Our provincial floral emblem since 1937, the distinctive three-leaved, three-petal trillium has become one of our provinces most well-known and beloved symbols.
The white blossom of the trillium is associated with peace and hope. Since 1964, the trillium has also served as the official logo for the government of Ontario. Over the ensuing four decades the trillium has become a universal symbol and seal for governments of all three major political parties.
While an indelible part of our natural heritage, the trillium's future remains a considerable source of concern. Trilliums take over 15 years to mature, and quickly die should their distinctive three leaves (their only food source) get picked. Many jurisdictions have passed laws preventing the picking of trilliums, and the preservation of this flower, and all it stands for, remains a laudable goal of conservation minded Ontarians province wide.
COAT OF ARMS HISTORY:
The Coat of Arms of Ontario was granted by Royal Warrant of Queen Victoria on 26 May 1868. The Shield of Arms consists of three golden maple leaves on a green background.
Above the leaves is the red and white Cross of St. George. It represents the English heritage of many Ontarians and also refers to King George III, who ruled England when Ontario became a colony. The golden maple leaves represent Ontario. Quebec uses green leaves, and Canada red ones.
So there you have it. The three maple leaves have been a symbol of this province for roughly 100 years longer than the trillium, though to be fair the trillium seems to be more recognized among the public because of its inclusion on all government letters. Maybe a new flag would need to include both.