During the first months of a tank being set up, it is common for the water parameters to fluctuate. This is especially true during the first month or so when a tank normally goes through the nitrogen cycle. But even after this, algae blooms work their way through as a tank matures. All of this can stress an anemone and cause it not to survive (this is true with a lot of corals/fish too).
Anemones are a super-long lived creature in the wild, but in lots of cases tend to live only a slim fraction of that in captivity. So by providing a nice mature and stable tank, you can increase the likelihood of the anemone surviving.