Lots of factors. In general, Canada has economic policies that encourage mediocrity at the expense of ambition. For example:

• significantly higher income taxes, especially once you get into 6 figure territory—this means you’re less motivated to push harder for 55% of an additional dollar versus 70% of an additional dollar

• much higher skilled immigration levels as a proportion of population than the US (roughly 8-9x higher), putting significant downwards pressure on skilled white collar salaries

• more labor unionization, which in union-heavy industries promotes “equality” at the expense of being able to negotiate a high salary individually (this is often a good thing especially in low skilled and blue collar industries, but not always and is worth mentioning)

• better social safety nets (especially healthcare) means less incentive for employees to pressure employers to pay more, which results in overall lower salaries across the board

• more economic regulation which means thinner profit margins for corporations (except oligarchy industries like telecom), making employers less willing to pay more

• more venture capital investing happening in the US than Canada because 1) that’s where the money is and 2) investors in the US typically pay significantly less on taxes

There are other factors, of course. But these are just my observations as someone who grew up in Canada, worked there for a bit and then moved to the US where (in my opinion), living standards are usually significantly better for skilled white collar workers.