First, why did we use the partitive article "de l’" instead of the indefinite articles "des" for the word "ananas".

You can use either the partitive or the indefinite articles here but it makes more sense to use the partitive if you didn’t eat whole ananas but chunks / slices of them, probably the most common case. That would even be no choice for bigger stuff like j’ai mangé du bœuf tous les jours (I ate beef everyday) and never j’ai mangé des bœufs tous les jours (I ate oxen everyday) unless you are a T-Rex ;-). Moreover, the English sentence doesn’t use the plural so doesn’t read I ate pineapples everyday…

I don’t understand is the usage of the passé composé instead of the imparfait; as it’s a past habit.

Both the imparfait and the passé composé are valid French. There have been many questions around this choice here.