You are asking why they have more movements. They have more movements because there is more demand and more airlines willing to endure the extensive delays that operating out of a saturated airport entail.
What you are I think trying to ask, is why is the movement rate higher at US airports. Correct? Most airports have about an 18 hour day: 0600-2400 give or take and within those times there are peaks and troughs.
Also your movement totals are a little off.
YYZ in 2009 was 407,724, YVR 313,984 and YYC 233,145. LGA in comparison was 354,048.
This will probably turn into a weighty post, but there is no real way to quickly explain this.
The difference in movement rates comes down mainly to the rules the controllers have to apply. Yes there are differences which enable US controllers move more traffic per hour than us Canadians. We're not trying to delay traffic and no controller can be a cowboy and get away with it.
I'll try and give a few examples of the differences with YYZ as the Canadian example as that's where I work.
Departure separations:
In YYZ aircraft on the same SID from the same runway have to be separated by 3 miles when transferred to Departure. This is the same in good weather or bad. Notice how big the delays get for LGA, EWR and JFK when the weather is less than 3 miles/1000'. That doesn't happen here as badly.
In YYZ jet aircraft, other than a few specified examples (CRJ1/2s, E135/145s and some corporate types) cannot be turned off the SID before 3600' except to avoid weather. This means that we are providing 3 miles in trail unless we have a turnable jet or a prop in the mix, which can be turned.
In the US there is a provision for separating subsequent departures by 1 mile and 15 degrees. So as soon as one aircraft lifts off the next can roll as long as there is 15 degrees divergence. This is way more efficient than 3 miles in trail. Visual separation of departures is still not an accepted procedure Canada-wide, although arrivals can and are routinely visually separated.
When YYZ is using only one departure runway, i.e. 33R when landing 33L, this 3 mile requirement really hurts. Especially since it takes longer to establish 3 miles as there will be at least a 25kt headwind.
If you decide you'll fly a VNAP-A, this delays all aircraft behind you by about 15 seconds per VNAP-A aircraft, as we have to wait longer before the next departure since your speed over the ground is reduced flying that profile and it takes longer to get the 3 miles. If we fail to provide 3 miles to Departure, or some other form of separation, it is considered an OI. We are treading a fine line between efficiency and a deal. You'll have to forgive us if we are a little cautious sometimes, especially with high performance types.
Runway occupancy:
HIRO is advertised almost 24/7 at YYZ, but runway occupancy times (ROT) are sometimes brutally long both from arriving and departing aircraft. We have annual surveys of ROT times and the results are startling, with times in excess of 70 seconds recorded. 2.5 mile arrival spacing is authorized on runways where ROT is 50 seconds or less. Not all runways at YYZ qualify due to the location of the exits but on those that do, we really need you to go into the exits quickly.
Time taken to line up is sometimes too long also. ROT here should also be 50 seconds or less. Have you ever received a take off clearance on a busy runway and you're not yet on the centreline? That means you're wasting space!
Are ROTs in the US consistently less? Probably, but I have no evidence to prove it.
Airport layout:
YYZ is a mess! Its not designed for the efficient movement of traffic. There are runway crossings required on many taxy routes, aircraft are often required to taxy to the furthest runway due to the standard split of traffic. When the 15s or 33s are in use this only gets worse. Don't even mention de-icing.
It would be great if everyone could depart from the closest runway, land on the runway closest to parking, but that just isn't possible much of the time.
I've rambled on long enough.