Language is very much "Use it or Lose it"
The problem is making it actually useful in daily life for most people.
They stopped teaching French at the schools I went to in Saskatchewan because all it did was cost money with very little if any results, the majoity of people didn't retain it, it was phased out of school when I was young and all I remeber is the Turkey, and I don't even know how to spell it in french, La dinde i think
It doesn't stick because there is no reason to speak it over the majority language in a region, There are few advantages for the average person to learn french, Doctorwett gave some "reasons" but those are not going to "click" with people, they aren't good enough to keep people speaking another language.
Millions of people in Canada have learned french at some point in there life, however the majority of those people had nothing to apply those skills to and now most are lucky if they can read a few lines on a package of noodles, let alone have a conversation or read a book in French.
And sure someone will say "im the exception" and you probably are the exception you learned french in some English heavy area and speak it often, but the several million others who also did that at some point learned French know almost none of it because it has no usage in their daily life and have no time to upkeep a language they never have to use.
When it comes to Language, you can't think of "just" Canada, you have to think of the 400 million+ English speakers in North America Versus the 12 million french speakers in all of north America,
Work, School, Home TV, Radio, Internet, Books, games, movies, and culture in general, have a majority of their base centered on English within Canada, When was the last time there was a big name french movie, or TV show? or video game, or even a book? not to mention the majority of jobs people do in Canada do not require french by any means, most of those are niche jobs and or Government positions outside of already french dominated regions.
The key is making a language useful for the average person in their daily life without requiring it to be a task that requires constant upkeep.
At the current rate of decline unless you can make French a useful language within the rest of Canada, it'll be almost gone by the end of this century, some estimates put the end of a sizable French speaking population in the 2050's-2060's.
You can continue to teach children French, but unless you can actually give everyone a reason to countinue using it outside of school it will do nothing.