This is a really good question. The 250g limit was a set from some, what I would consider flawed, experiments with collision testing on aircraft. While in Canada indeed it's true that under 250grams does not require registration, you still have to adhere to several rules. The big one is 900.06 which basically states that you have to fly in a safe manner. You still shouldn't fly over people, cars, etc even with small drones. A friend of mine had the RCMP show up while he was flying his 100g whoop in his front yard. He lives 2 miles from the nearest airport, and has 100 ft trees around his property, yet he was reported to the tower by a pilot at 500 feet. The limit of 250g was I believe fairly arbitrary and like all printed regulations, an amendment is just one idiot away. At a recent meeting with a MAAC representative, he did say that the limit is being re-visited by TC so I would not be surprised to see the registration requirement being amended so that they all have to be registered. The other factor that is perhaps fueling this is that of the estimated 140,000 RPAS pilots (and drones) in Canada that should be registered, only around 32000 have bothered to get their registration (29000 basic, and 3000 advanced, but this is misleading because almost all advanced pilots got their basic first, so the real number is probably closer to 30000). Either their estimates were off, or there are a lot of drones collecting dust and people who just gave up the hobby. If their estimates are correct, only 20% or so have registered. I suspect that latter as it seems to be harder to sell drones, at least in my area, with a lot of folks I personally now that fly, have tried to get rid of their drones with little success.
TC also estimated that there are 200,000 drones in Canada that should have been registered, but only 38,000 have. As a matter of interest, 81% or so of the registered drones are DJI, and yes, even mini's have been registered. Of 245 different manufactures, only 4 represent 86% of all registered drones.
The bottom line is that nothing is static and as several countries have done away with the 250g limit, or never had a limit for registration, I wouldn't be surprised if this was done away with by TC, and in the US by the FAA sometime in the near future.
The statistics I mention are directly from TC, but if you want more detail, Don Joyce made a nice video outlining the numbers:
if you're interested.
cheers
ks