The 6 month priorities are due to PSC, Facebook's performance review cycle. Work that took more than a half effectively doesn't count for bonuses or promotions and could possibly get you fired, so it's disincentivized.
As of 2022, PSC is shifting to every 12 months not 6 in an effort to better support longer-term projects and thinking. Promotions will still be assessed every 6 months.
FB policy says they want to reward results and not effort. But it makes it hard to reward people for making progress on things that can't be done in 6 months.
It also makes for shitty service quality around the end of June and December, as people rush to push things to prod. (I've heard this may have changed, but it's hard to notice good service quality).