Because this is a brand new coinage that nobody ever bothered us with until recently:
It’s also because no dictionary ever enumerates all possible words produced by applying productive derivational suffixes to base words. And few spellcheckers work that way, either, even though they should.
The scattered occurrences in the historical record do not yet justify wasting limited space for something everyone knows what means.
I strongly suggest checking occurrences in Google Scholar if you want to see acceptable scholarly uses of your term. Check other corpora as well; notice how Google Ngrams reports zero occurrences of implementational in their English Fiction corpus.
Then again, the OED2 does attest to nearly two hundred words like it, many of which you surely have not yet encountered before. So I imagine they’ll eventually get around to yours, too. :)