Credit: 3++ Armies Played Poll Analysis
There we are, #4 on that list. However, Space Marines is the catch-all beginning player army that represents more chapters than any other codex generally. I am quite surprised to see Eldar doing so strongly, but that may be due to 4th Edition popularity and people didn't want to part with their lovingly painted models. The Blood Angels' twin from second edition, Dark Angels, are far behind in popularity yet may shoot up again once their new codex drops shortly and through new players buying the starter pack (well played, GW). I'm not going to lie and say I'm not shocked to see Tau ahead of Orks, but maybe Ork players are more dedicated to them and play them more as a main army.
I've read before that the Blood Angels are the most popular single chapter of the Adeptus Astartes. Having a red army helps, but I think there is something about having Assault Squads as troops and Death Company that really drives interest in this chapter along with a focus on close combat. We can all imagine hurtling ourselves through the air with our jump packs to come crashing down with Thunder Hammer to smash our opponent or Lightning Claws to rip them to shreds. Every army needs some fire support though, and Devastators are an excellent choice to neutralize threats. Jumpers and Devastators make a fun combination to play on the table and interesting to customize and paint.