Can you explain why “Linux from Scratch” instead of Gentoo? Which community is bigger?
Gladly and thanks for asking!
I have both Gentoo (now at release 2.1) and Linux from Scratch, hereafter LFS, on my system -- GRUB is a beautiful thing since it makes booting into different environments a breeze (also Solaris is present here on my box as are OpenBSD and FreeBSD).
Gentoo is much like LFS in that both are source-based systems. They also share the trait of having excellent documentation since the presumption is that the user is very hands on and actively involved with the guts of the operating system and the applications as well as system administration.
Where they differ is that Gentoo packages all of the compilation steps into a tool known as "emerge". For instance, to get the latest Firefox on a Gentoo system, just execute "emerge firefox" at the command prompt. It then takes care of downloading the source, chasing down the dependencies and then compiling everything. In contrast, with LFS, those same steps are up to you to perform manually.
For me personally, I have a slight favoritism for LFS because I prefer to be be involved with every step of the process, even if it's some additional work. I guess a rather strained analogy is that with Gentoo, someone delivers all of the ingredients to your door but you still cook the meal. With LFS, you have to grow all of the crops and maintain the animals (and do the slaughtering). :-)
I failed to answer that part of your question. As a wild guess, I think both are niche markets in the grand scheme of things compared to the huge Ubuntu and Fedora slices of Linux.
My opinion is that the LFS partisans are a just a tad more informed than Gentoo. That's probably because most LFS people are more used to solving their own problems since they are DIYers to the extreme. There's a bit more hand-holding with Gentoo because some of the steps are abstracted out with the "emerge" tool.
Most (by no means all) Windows users would probably consider either LFS or Gentoo to be utter nonsense. Sure, the computer is a tool but with LFS or Gentoo, we're also the toolmaker!