The reason is quite obvious if you know Spain's History.
Given that France invaded Spain in 1807, the military made it necessary to have a different gauge from France. Not only that, the train by the coast was also forbidden in some places as a naval bombardment could disrupt communications in case of war.
Spain has lots of mountains with a large plateau over 700 meters high and the coast is usually way lower so it makes sense to transport things by the coast.
Given that France invaded Spain in 1807, the military made it necessary to have a different gauge from France. Not only that, the train by the coast was also forbidden in some places as a naval bombardment could disrupt communications in case of war.
Spain has lots of mountains with a large plateau over 700 meters high and the coast is usually way lower so it makes sense to transport things by the coast.