Newton wrote in Latin because he was, famously, an arrogent bellend. The rest of the sensible scientific community had long since swiched to living langauges.
Newton also wrote Opticks in English, perhaps because he was aiming at a more mass market audience.
"Leibniz wrote mainly in three languages: scholastic Latin, French and German."
https://en.wikipedia.org/wiki/Gottfried_Wilhelm_Leibniz
"Many of his writings were in Latin"
https://en.wikipedia.org/wiki/Carl_Linnaeus
Newton wrote in Latin because he was, famously, an arrogent bellend. The rest of the sensible scientific community had long since swiched to living langauges.