Ratskinner
Adventurer
I dont know, the low birth rate thing makes a good deal of sense to me. I think many of shidaku's proposals also make good explanations. Population size is definitely going to be a factor in a society's ability to compete with other groups. Wtih elves, the issue they face is, living as long as they do, they may be more cautious than short lived races about risk.
I think the "cautious" thing is where I tend to fall. I also think its rather a good explanation for the "low birth rate" thing as well. Even in humans, the birth rate lowers in societies with higher standards of living and longer lifespans for women. Elves, looking down the barrel of hundreds of years, have more cause to worry about things like sustainability, relationships, and quality of life, then tack on a few more hundred years for your kids....seems like that caution would easily extend to making baby elves. (It also explains why, in the standard-issue Tolkienish model, Elves burst onto the early world, and then come to a screaching halt....they learned.)