Flight plans are computer generated by flight operations. Westerly routings to the canaries can require oceanic clearances and can be a bit more complicated re aircraft equipment etc. On the other hand they may well avoid slot delays as well as benefiting from better winds.
As captain you rarely have much input on the routing other than rejecting the flight plan for some problem like aircraft inoperative items or very occasionally fuel reserves versus destination weather.
The Canaries usually have great weather, but on the few occasions they don’t then it can be that all the airports are affected and you end up with diversions all over the place.