All lines between two points on a Mercator projection are true bearings.

If one place is North East of another on a Mercator projection, if you sail North East from the second place, you will reach the first place.

(Of course you need to make the correction from true bearings to compass bearings caused by the fact that the magnetic North Pole is not at the same place as the actual North Pole. If you make that correction however, you can set a compass course and reach your destination.)

So, the Mercator Projection was the favored map projection because it was the most useful map projection, especially for nautical navigation, which was one of the main uses of maps which covered large geographies. Because the Mercator Projection was the most useful projection, it was the most commonly produced type of map, and the type of map most people got used to.

The drawback of the Mercator Projection is that it exaggerates the size of features in the high latitudes. All flat maps have some problems, as it is impossible to totally accurately depict a three dimension spherical world on a flat two dimensional piece of paper.

The accurate bearings that could be read off the Mercator Projection were very useful and more than compensated for the various drawbacks of that depiction of the world

No, it was favoured because it could accurately interpret distances, which caused its popularity in naval transport.

Aside of that, the Mercator projection is old and became popular long before the formation of the USSR and even before the British took possession of Canada.

Mercator is probably the easiest projection to construct (other than Plate-Carrée), a serious consideration before calculators. It also gives comparatively more room for features and placenames in Northern Europe, the location of most of the mapmakers and mapreaders at the time. Never attribute to avarice what can be explained by sloth.