It's because they want to take advantage of the Canadian market while they still can. We didn't even have unlimited plans until last year. They're slowly rolling them out because they pretty much have to at this point, we are way behind in terms of plans.
Now to continue to capitalize off unlimited plans they chose to cap the high speed at certain thresholds like 10gb 20gb 30gb etc, and once you go over they reduce your speed. This is to try and lure people into buying a more expensive plan.
A 10gb 4g unlimited plan might be $60, but a 20gb might be $75. Most people want high speed data not old 3g lte technology.
Its just a play by big telecom to make more money. Eventually all plans will switch to unlimited high speed but I don't expect that for a few more years, and then prices will drop incrementally over time until they find a new reason to charge more money, like an even faster network.