Because it's easier to learn this game as a dps, than as a tank. Atleast thats my opinion, speaking from a bit of experience.
I think it depends really. A DPS can generally learn mechanics by watching as they progress through the content, slowly, over the course of a few attempts, but there's just some stuff that a DPS never really sees. A tank has to learn by just jumping in feet first and sees or is on the receiving end of pretty much everything. Some people learn easily by jumping in feet first and getting bombarded by a lot of stuff rather than having to take it slow and missing a lot of little things, like attack/damage types.
The more important aspect is actually in the general forgiveness of the community concerning someone *learning* the content. DPS generally get a lot of leeway because a bad DPS doesn't really screw a group over. Tanks generally get *very little* leeway because a bad tank will cause *lots* of problems. If you find a group with patience and that is willing to explain stuff, you can do very well as a new tank. If you *don't*, then you'll have a terrible time of things as you're routinely booted because people just don't want to deal with you at that moment (or ever).