There are a few reasons that I get writers block.
First is the completely elusive cause that no one understands, the sudden disappearance of all sensible content from the conscious mind. One moment there are dozens of thoughts and ideas scurrying around your mind—the next they've vanished, leaving no remnant.
Second is the world of distractions. Writing in my house means that the house needs to be clean and neat (dishes washed, clothes folded, and table cleared of papers and clutter), otherwise it's a constant battle between my desire for a clean space and my will to write. Writing in a public space means that there are dozens of people for me to observe, and my mind would always rather wander than work.
The third is like the rusted hinge, the scuff free shoes, the ten year old car with 2,000 miles. It's hard to do anything cold turkey—diet, exercise, sing, and write. I write every day, partially because I love it but also because if I don't, when I come back to the keyboard all that greets me is a blank screen and the crickets. And the crickets. And the crickets.
I'm learning to write with dogged persistence. Sometimes it's tiring, often it's hard, but it's consistently rewarding.