Simply put, it was a matter of timing, location, and aggressive marketing. The Canadian army kept ww2-era Centurions all the way into the 70s. Even with running upgrades, their reliability dropped to the point that they became desperate for a replacement in 1975 in order to fulfill their NATO obligations in West Germany. The U.S. army was developing the M1, but those weren't going to be ready until the late 1970s, and Canada would have had to wait a few additional years before acquisition.

Seeing that a large part of Canada's MBTs were to be deployed in West Germany, and sensing an opportunity, West Germany proposed that they lend a few dozen units from the W.German army to the Canadian army until Canada could get their own Leopard 1 tanks. Otherwise the Canadians will have had to wait a few years before production could be fulfilled and the tanks delivered. This arrangement worked out well for everybody.

I assumed you were asking about the Leo 1, as the newer Leo tanks may have been purchased too recently.

Here's a nice recollection in more detail: http://canadianmilitaryhistory.ca/from-centurion-to-leopard-1a2-by-frank-maas/

When did Commonwealth nations cease to simply use British war materiel?

Meta question: Why are there so many Canadian military questions lately? Not that I'm complaining, but is there a common reason that inspired these questions?

Side question: Where weapon exports to NATO countries limited in any way after Germany was allowed to produce weapons again?