This earlier question Is using “fruits” as the plural of “fruit” acceptable? has a number of answers as to whether fruit can be in the plural form fruits.
The consensus of the answers there seems to be to use the singular fruit in general and use the plural fruits only to describe "different varieties/kinds of fruit".
But I think there's more to it than that.
I'm not sure if fruit in this phrase necessarily denotes different varieties/kinds of fruit. But even if it does, the same phrase shows more hits for the singular fruit in British English.
Of course, the same phrase shows substantially more hits for the plural fruits in American English.
Now, returning to the original Ngram, it seems to me that the more hits for the plural fruits is apparently due to the fact that the gap between the two uses is larger in American English than in British English, regardless of whether fruit in the phrase actually refers to "different varieties/kinds of fruits".
Am I on to something or am I mistaken?
For those AE speakers who say Americans never "say" fruits regardless of the prevalent proof that they "write" fruits, here's some proof saying otherwise:
(1) Study: Eat 10 daily servings of fruits and vegetables by Fox News
(2) Dr. Campbell: Eating more fruits and vegetables can prolong life by CBS North Carolina
As you can see they speak American English in both these news reports and they talk about the same research paper.
Now, here's another video addressing the same research paper but "This Morning" is a British channel and they apparently speak British English. And I notice they keep saying "fruit and veggies" never "fruits":
(3) Do We Really Need to Eat 10 Portions of Fruit and Veg a Day? by This Morning