both of which could've fielded another division to take the beachhead?

Largely because they really couldn't. Especially so in the case for the British. For logistical reasons it made perfect sense to allow the relatively fresh - but experienced - Canadian forces shoulder some of the burden.

Canada, and Poland (who ship the 1st Armored Division to the coast in August) have the distinction of being the only forces to consist entirely of volunteers in the battle zone.

The British were reaching the end of their manpower tether, by the end of the campaign in Normandy, whole divisions would be disbanded and their men merged into other divisions. This should stress to you just how serious a manpower crisis was looming for the Island nation. Among its dominions and commonwealth affiliates, Canada was certainly the freshest in 1944. We had seen combat: by that time in Hong Kong, an abortive campaign in the Aleutians, various raids along the Atlantic Wall, the invasion of Sicily and Italy. Despite this, we were, logistically, in a far better position to take over the manpower burden for the British then the ANZAC forces. NZ had its forces already tied down in Italy, remaining forces headed back to the Far East after the successful conclusion of fighting in North Africa. They were far from home and not a populous nation by any stretch of the imagination. These same problems are equally applicable to Australia. By contrast Canada was 'only' across the Atlantic, was a breadbasket, somewhat more populous and had a robust merchant marine compared to its smaller sister Dominions.

The US could easily have mobilized many more men, but not without beginning to strain their industrial workforce to a significant extent. The same industrial workforce that was at that point in time helping to fight a two front war, support the Commonwealth nations, re-arm the Free-French, and support the USSR and Nationalist China. A reduction in industrial capacity would've been highly unfavorable to the situation. So while it seems rather 'easy' for the US to simply mobilize another division, for an undertaking such as the invasion of Europe, it would ripple out to alot of secondary and tertiary factors. Who is to transport them? The naval component must now find that manpower, it may require further recruitment - and extensive and thorough training for co-ordinating a beach assault-, it may require the formation of additional corps-level units to support the larger formation, medical battalions, quartermaster depots, tactical air support, signals, tank destroyers, anti-aircraft artillery. The list goes on and on, suffice to say, the amount of support and manpower to keep an Infantry division in combat and replenished in both manpower and ammunition is significant. It also would've extended the area of responsibility for General Bradley significantly, when the Canadian army had its own command structure that was ready to take control of a relatively narrow front.