What a lot of idiots like Animats just don't get about Arduino is that Arduino managed to get a fun package out on the market anyone can get started with and can get started with without having to rely on the type of people who snort derisively at anyone who cannot tell resistors apart by smell.
Just buy a kit and you can experiment for fun in a way that is fun and does not involve the need to first lean how to measure voltages until you want to.
Yes there are countless other systems out there, both more powerful and invariable connected to communities that are exclusive rather then inclusive. Ask on an Arduino forum for stuff anyone knows about as beginner and you get an answer. It is the same reason Linux beat BSD and PHP beats say Java or Ruby. Not because they are necessarly better products but because more people can get started with them.
Plenty of people who mess with these kit robots just want to mess about, use them as toys, have a bit of fun. They don't need massive parallel processing or sensors with miles of range, they do not dream of making a spy bot. They want to have a toy that follows their hand or make their own beer cannon or just see some leds light up. I know to anyone who has a full soldering stations it might sound incredible but for the majority, first having to spend a weekend soldering a kit together is NOT what they want if they buy a diy robot kit. They want to spend maybe an hour at most and then have something that moves. From there they may or may not advance. 2D SLAM? Jezus Christ Animats, just how sad are you as a human being?
Most people don't want to instantly have to develop the next generation mars rover. They want to have some fun! PLAY. If you like remote control aircraft, you start with a simpel plastic model and have fun and MAYBE someday you will go further OR NOT! Maybe start with a click and play model train set like Kato and MAYBE one day move on to making your own tracks from scratch. MAYBE. But from beginner to expert there is a lot of room for simply having fun, for enjoying toys at the level you are comfortable at and are willing to spend the time learning. And Arduino sits there are the beginner entry, middle level, open and welcoming and not demanding people first follow a 4yr electronics course to get anything done and because of that it is a massive hit. And it is getting people involved in messing with electronics who never would have before and it is GREAT!
And in the background are the forever alone losers trying to point out there more powerful toys with impossible to read manuals and secret society forums where you can only post if you first lurked for two years as an apprentice.
It is kinda sad in a way because it is just basic business sense. You want to sell something, you got to make it accesible. Arduino is the next step up from Lego Mindstorm. They got what made Mindstorm such a gigantic hit, an open accessible friendly platform where you can either remain as a fun loving beginner or use as a stepping stone.
And forever aloners like Animats will be crying "but we got more cpu power" as the rest of the planet has fun.