The older ESDs (most of the FR, Dark Sun, Al-Qadim and I suspect Dragonlance) are really neat, because they put a lot of time into the scanning process. The text on the page is actually text, not just a picture that looks like text. The files are pretty small (e.g. Forgotten Realms Adventures using the old process is ~6 meg, Tome of Magic using the newer one is ~25 meg, both books are about the same size physically), mainly on account of the text/picture issue.
I think the decrease in quality happened before Bastion took over the process, however, and the reason was of course the almighty dollar - the older process took much more time, and time = money.