Agree with Glaysher. All magic comes with a price, and the use of the Druid Sleep has one of the heftier price tags. Bremen and Allanon used the sleep because they knew Brona would return, and there must be someone there to face him. Cogline used the Sleep for similar reasons regarding the Shadowen (and I believe there was a line in there somewhere about Allanon not being finished with him) and to help Walker Boh return the Druids to the Four Lands.
Then Walker used the Sleep because he was the last of the Druids, and had to prolong his life until an apprentice/replacement could be found, and his charge could be fulfilled.
Once Grianne Ohmsford re-established the Order, there was little use for the Sleep, and so its use eventually diminished as it was no longer the necessity it had been in the days of Bremen and then Allanon. So naturally, Druids began living normal (or closer to normal) lifespans as they were no longer using the Sleep to prolong their lives.