Because it was built in 1940...
I'm sort of kidding. :-) Still, there's a fair chance there's some lead either in the house or more likely in the supply line coming into the house.
Metal shortages during WWII did lead to various substitutes. There was a lot of steel/iron pipe in use then, except that iron became very valuable in the America's for the war effort (especially in the U.S., more than in Canada I suppose).
But bear in mind that lead solder was used to fit residential copper pipes until the 1990's, so even if you have copper pipes that were installed after the 1940's, you're likely to have some lead in your plumbing.
You're just going to have to inspect the pipes, or have an inspector check them out.
And as the other answer suggests, if you're concerned, get the water tested.