Because there were hundreds of episodes to adapt when they started off?
And as they came closer to the manga the pacing started to suffer. It ain't astrophysics.
Once again: Nonsense.
Let's take episode 270 (Luffy vs Blueno). This aired on 07/03/2006 and adapted one and a half chapters. Those chapters came out on 09/25/2005 and 10/03/2005.
We can go even further back to the start of Water 7, episode 229. This aired on 04/17/2005 and also adapted a bit more than one and a half chapters. Those chapters came out on 05/31/2004 and 06/07/2004.
So they were even closer to the manga then they are now and their pacing of one and a half chapters per episode continued.
The major difference that allowed them to keep up such a nice pacing?
1) They actually put in some well timed stretches of decent enough filler episodes.
2) They had a rhythm of 3 episodes and then a 1 week break back then (you can look it up).
That's enough. Unfortunately they scratched both of those points at some point and the quality of the anime pacing suffered greatly.
PS: Funny coincidence that you mention astrophysics while talking to an actual astrophysicist^^
Actually, it was more like late Enies Lobby(because something very obvious happened, ie, they caught up to the source material a significant amount).
Nope, read my post above.
At the Post Enies Lobby Arc they still had a pacing of one and a half (sometimes even two) chapters per episode.
Take episode 325 (Ace vs BB) for example. This episode aired on 09/23/2007 and adapted a bit more than one and a half chapters, which originally came out on 01/06/2007 and 01/22/2007.
So they were still 8 months behind the manga (compare that to the start of Water 7, where they were about 10 months behind the source material).
But what did they (wisely) do?
They put in a nice little Icehunter filler arc (11 episodes) after Post Enies Lobby and suddenly they were almost a year behind the manga again.
It was a perfect synergy.
But then they changed their model and fucked everything up.