Both grand- and great- seem to be translating the French grand-, as in grand-oncle and so on. French uses grand- consistently for the upward direction, and petit- for the reverse, as in petit-fils (grandson). In Latin your great-uncle is patruus magnus if he is on your father's side, and avunculus magnus on your mother's side; magnus, like French grand, can mean both "big" and "important". The root of our problem is that there are two ways to receive that into English: grand via Anglo-Norman graunt, Old French grant; and the Germanic great.
It seems that grandsire and grandam were the earliest English uses (appearing in the 13th century) which are pretty obviously based on the French. On the other hand, great-uncle and grand-uncle are both attested from the 15th century:
his grete Uncle H. Cardinal of England (Rolls of Parliament V.438, 1438)
his graunt oncle Henry cardinalle of Englande (Book of Noblesse, 1475)
In modern times I think most people are familiar with the "rule" that grand- means a difference of two generations, and repetitions of great- can be used for longer distances. This would suggest grand-uncle as the preferred form but in fact, as your chart shows, great-uncle is more common. Perhaps what is going on is that both words are possible English forms of grand-oncle, but for some reason grand-uncle does not feel appropriate, leaving the way clear for great-uncle to become standard. They are both used, and have a long history - but one is more popular. Pure speculation:
If my grandfather is my father's father, then my grand-uncle should be my uncle's father. But that is my grandfather. So grand-uncle is confusing.
grand- is reserved for the direct line of descent, so it feels wrong to use it for people who are off to the side