I think that, to be frank, it's a combination of (1) a curriculum developed before it was clear how ubiquitous linear algebra would become, and (2) the fact that it's a lot easier to come up with a standardized assessment for algorithmic calculus than for linear algebra, precisely because linear algebra is both conceptual and proof-based in a way that has been squeezed out of algorithmic calculus.

(I use algorithmic calculus to describe the high-school subject, and distinguish it from what in American universities is usually called "analysis," where one finally has the chance to make the acquaintance of the conceptual and proof-based aspects squeezed out of algorithmic calculus.)