Sorry it took a while to respond, just trying to catch up after vacation.
It's a good question, although I would also point out that they say 15/10 (Or 1), so that suggest technology dependence. The short answer would be that upstream has historically been of less interest to the Internet user. Certainly today we are seeing a shift in that line of thinking, and we do talk about adjusting speeds. However, if it was as easy as just turning the dial up a few notches we'd probably be moving faster.
DSL by it's nature is mostly asymmetrical (which is where we started with ADSL). We do have some symmetrical services, but that's on G.SHDSL and really for businesses. It doesn't have to be as wide a difference in symmetry for VDSL2, but if you start to shift from downstream to upstream, you will have trade offs. All broadband, whether DSL or cable, is limited by the frequency bandwidth you have available in the final mile. So if we are consuming the bulk of the frequency to deliver content down, we have less for upstream. Alternatively you could increase the width of the spectrum you want to use, but then you will sacrifice reach.
Last is limitations of the premise equipment. Having been involved in many iterations of routers, and challenging staff to squeeze what they can from it, I often wonder just what other companies are using. The 5168N that we currently purchase is a pretty good router, and we can certainly pump some serious up and downstream through it. Today the bigger challenge is getting something with a CPU that handle the number of connections we make to it, particularly wireless. In looking into trouble reports (we typically see the chronic or unique ones), it's crazy how many devices we connect to our home routers. I know with just my wife and I at home we'll have two phones, two tablets, a laptop, printer, Xbox (yes, I Halo), phablet, Chromecast....all simultaneously connected. So we really need to consider the strain we are going to put on the RG when we dabble with speeds.
Canada as a whole is pretty consistent, but there will always be places where a company is trying the waters or maybe carving out a segment they feel is underserved or under represented. Sometimes that works, sometimes it doesn't. Have a read of this CBC Technology article. While 2 years old it paints a pretty accurate picture (although I'd debate his statement about reluctance to turn the technology loose).