*waves hand*
Oropher had come among them with only a handful of Sindar, and they were soon merged with the Silvan Elves, adopting their language and taking names of Silvan form and style. This they did deliberately; for they (and other similar adventurers forgotten in the legends or only briefly named) came from Doriath after its ruin and had no desire to leave Middle-earth, nor to be merged with the other Sindar of Beleriand, dominated by the Noldorin Exiles for whom the folk of Doriath had no great love. They wished indeed to become Silvan folk and to return, as they said, to the simple life natural to the Elves before the invitation of the Valar had disturbed it.
[...]
The Silvan Elves hid themselves in woodland fastnesses beyond the Misty Mountains, and became small and scattered people, hardly to be distinguished from Avari; but they still remembered that they were in origin Eldar, members of the Third Clan, and they welcomed those of the Noldor and especially the Sindar who did not pass over the Sea but migrated eastward [i.e. at the beginning of the Second Age]. Under the leadership of these they became again ordered folk and increased in wisdom. Thranduil father of Legolas of the Nine Walkers was Sindarin, and that tongue was used in his house, though not by all his folk
This is from a couple of different drafts and there are contradictions, but the picture outlined suggests that Oropher may have had a grudge against the dwarves and the noldor for their respective roles in the destruction of Doriath and that he made a serious attempt to undo the elven advances of the First Age out of a desire for a more primitive life.
Then, when he joined the Last Alliance, the backwardness of his people cost them terribly, he died, Thranduil became king and over the centuries of the Third Age he seems to have walked back his father's policies.