Well, I can't answer for all the Northerners, but in our case it all boils down to the grazing season and climate.
Grazing Season
Honey and I both have allotments in our local community pasture. We pay for our grazing from turnout which is the weekend closest to June 1, up to when we bring them out...usually the third weekend in October.
To prevent predator problems, one of the regulations that the grazing association members drew up is that no cows are turned out into pasture if there is a chance of them calving on the range.
So, basically, you want to have all your calves on the ground at least a couple weeks before turnout, so that all the calves are old enough to trail out and not wander off from their dams.
Now in our particular pasture, all the bulls are turned out onto grass at the same time as the cows. Due to the fact that the areas are only subdivided into large general areas (termed 'sections',but not as in the land measurement) there will be usually three or more different members cattle herds in each 'section'.
This means that even if you wanted to break a regulation and not turn your bulls out hoping for later calving dates, you're outta luck, as the other fellows bulls will be working on whichever cattle are handy. So basically, like it or lump it, with a June 1 exposure date to bulls, you're going to start calving second week in March. And if you haven't turned your bulls out with your cows before you turnout into the pasture, there's no guarantee that you'll be getting calves from your own bulls.
(All bulls do have to have proper breeding tests and registration papers, and must be dehorned - mind you all cows have to be dehorned as well - but that's another story for another day.)
Climate
Alrighty, so our climate is kind of chilly in the winter months, so if you're calving in the early part of the year, you know you're going to have to be vigilant about your checks in order to avoid having calfsicles. So if you start calving in February, in a perfect world one would hope to be pretty much done by the end of March/beginning of April.
Why not calve in April some might ask?
We know that keeping calves warm in Feb and Mar calving is the priority, but keeping them healthy in April and May can be a real challenge when faced with newborns.
I live in an area that has a humdinger of a spring...we call it "Break-up". What's that? Well, basically, it's when the frost comes out of the ground and the whole world turns into one big giant gumbo-mud-pie.
So now your barnyards become mireyards. Trying to feed hay sees you bog down the tractor three times from the hay yard to the cattle. Just keeping a pair of gumboots from being sucked off your feet in the goop is a big enough challenge, much less trying to find some clean areas to put your expecting cows and new calves. And then of course, all the warm weather and moisture combine to make a lovely cocktail called "Scour Soup", and once that little wonder starts rolling it can be nigh near impossible to keep up with doctoring sick calves.
It usually takes to about mid May here till you can almost not count on any more snowstorms, so by then things are generally getting firmer underfoot.
Now in a perfect world, Honey and I would own oodles of acres, and we'd calve 'em all on the grass starting in the middle of May. In the meanwhile, we just sort of make do.
Hope this at least sheds a glimmer of light on why some of us are busy with calving cows in February. ;-)
Take care.