Why is Canaria better than other alt layouts for Spanish? As a Spanish learner, Colemak-DH seems to be pretty good for both languages (although my usage is still overwhelmingly dominated by English).
Interestingly, Spanish might be easier to optimize as a solitary language if the punctuation keys are moved away from the vowel cluster. You can get insanely-low SFB scores. Spanish is a higher-syllable density language with better spelling conventions than English.
What some of the alt-Spanish layouts have done is just take an EN-optimized layout and add diacritics (and Canaria is not an exception here either, it just restored some ergo sanity to Spanish vs. pushing Q and J to bad spots like most EN ergo layouts do). For those who work entirely in Spanish most of the time, EN-ergo layouts are not ideal.