Why does Canada recommend a minimum rate of climb for ultralights of 300 ft/min?
When you fly gliders you discover it's quite common to run into air that's descending at 1-200 fpm, or "sink" in soaring-talk.  Descending air next to a thermal, or air descending due to downsloping terrain.  It's a lot more than that at times, but a couple hundred fpm is typical.  

On a day where there's any convection (with rising air, there is always equivalent descending air adjacent to it) you are in sink quite a lot.  A 300 fpm ROC provides a reasonable assurance that you will be still able to climb while in most (but certainly not all) conditions of descending air, barely.

If you have an ultralight that can only climb 100 fpm, this might be fine on a smooth day with stable air, if you're patient.  If there is any vertical motion in the air however, you have a problem.  The minimum climb rate requirement is an attempt by the regulator to ensure that that average ultralight buyer will have a machine that won't kill him because it couldn't even out-climb a bit of subsiding air at the end of the runway.