Why Greek morphemes over Latin, or Latin over Greek? *A Call to Lexicographers*
Solution 1:
It may be possible to know why Latin was chosen over Greek (or vice versa) in particular cases. Let's take hydraulic over "aqualic." The etymology of "hydraulic" is actually ὕδωρ (water) + αὐλός (pipe). The reason that we don't have the parallel word from Latin, "aquatubic," is that the Romans, who admired things Greek, including Greek engineering, had adopted the Greek version in their word for water engine, hydraulica machina. Remember that Latin was the European language of science and engineering into the 17th century.
Why "unity" and not "monity"? It's only a guess, but perhaps there was a collision of the latter with the variants of "monition," a word meaning a warning that came from French. After 1066, words of French origin, particularly legal terms (of which "monition" is one), had priority.
The courses of the "lexical streams" that flow into English are complicated. The Romans brought Latin directly to Britain over 150 years or so, starting with Caesar, who also inflicted Latin on the Gauls so the Normans could reinflict it via Old French on Britain a millennium later.
Some words, like "photography" aren't "natural" linguistic entities, but rather arbitrary "learned coinages," terms made up on the spot by inventors.
Solution 2:
Some reasons can include:
Community of origin. In particular we tend to have Latin a bit more often than Greek because over the last few centuries Latin has been better known.
Personal preference of coiner generally.
Perceived euphony in a particular case.
Matching related words; both heptagram and septagram can be found, but heptagram is more common among mathematicians as it then matches the other -gram shapes.
Avoiding collision with another word; We have the word television because telescope was already in common use for something else.
Solution 3:
Science, medicine, chemistry, biology, philosophy, mathematics, psychology and so on need a lot of scientific terms, so it is no wonder that Latin and Greek elements of word formation are used. There can't be a system in the creation of the terms in the various disciplines and often they are the creation of one individual scientist going by his personal preference. And as to time the development of those terms happened in various centuries. Sometimes simply words already existing in Greek or Latin were taken.
The lexicographers only register those words, they don't create them.