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We can see in the answers to Why would "breakfast" be translated as "lunch"? that, up until at least the end of s. XIX, almuerzo and desayuno were proper synonyms.
Nowadays, however, to most people they mean different meals (the most common distinction is, I think, that desayuno is had first thing in the morning, i.e. a breakfast, and almuerzo is kind of a second breakfast, closer to midday).
When did this distinction appear?
And why did we set on "desayuno" being the earlier one?
I can only answer the 2nd question. The verb desayuno is so common that we don't realize it is des-ayunar.
From the DLE:
ayunar
1. Abstenerse total o parcialmente de comer o beber.
In desayunar, 'des-' is the negative prefix (which also appears in the DLE, and it's interesting on its own). The first time you eat something, you stop the ayuno, so desayuno is necessarily the first meal of the day.
It's funny that we say "Hoy no he desayunado" all day long when we skip breakfast, when originally that sentence would only be valid until your first meal, which would then become your desayuno even if it's 6pm. However, nowadays it has obviously linked its meaning to "the meal of the early morning".
By the way, ayunar is "to fast" in English. When you eat something, you're breaking your fast. Hence "breakfast".
