Because under the current system, you can sue anyone who is "doing business" in a location. As long as you accept online sales, you are doing business anywhere. Add to that the attractiveness of bringing in lots of court fees, and some politicians in Texas got the idea to appoint judges that would be extremely patent friendly. They then brag about the high rate of settlements/favorable judgements and the speed in which the court will hear your case
Bottom line: marketing and politics created a forum for patent trolls to get a court to back up their extortion. Texas rakes in the court costs, the trolls rake in the settlement/judgement money, and everyone is happy....right?