I guess that there are multiple reasons.
The most common one is ignorance: most people don't know how tabs are supposed to be used as indentation and indentation ONLY, and how to set up an editor appropriately (and the shell, by the way) (see the other responses to your comment for proof).
The second is lazyness / non-confrontational behaviour: If you settle on tabs, you will have to educate all users about the correct usage, and have to bear with people that just insist on using tabs differently (see point 1). If you settle on space usage, that's about it.
The third reason is that some editors are stupid, because the implementation doesn't know that tabs are supposed to be used as indentation and indentation ONLY, and will (for example) insist on aligning multi-line arguments to a function just after the parenthesis using a liberal amount of tabs.
And then there are some language requirements that like to mess things up, like Makefile requiring tabs and other scripting languages pursuing a vendetta against tabs (like Python).
That being said, I will gladly educate people about tab usage, their ignorance is no excuse for using inferior solutions.