Why don't all flights use the North Atlantic Tracks (NATs)?
The answer doesn't state that half of the flights between western Europe and North America use the North Atlantic Oceanic Track System (OTS). It states that half of the flights over the North Atlantic use the OTS. 

Part of the flights over the North Atlantic don't cross between western Europe and North America and they don't use the tracks. Examples are flights to/from Iceland or the Azores. Also flights crossing between Ireland and the North Western point of Spain are entering the North Atlantic Region.  

Also part of the flights that do cross between western Europe and North America and vice versa may use random routing for various reason.

On some days the winds are such that favourable tracks to North America are within the Reykjavik control area, which allows for random routings. 

Some combinations of origin / destination are better served by avoiding the OTS area at all. For example flights between the Iberian peninsula and Florida are generally South of the OTS.