Because when it comes to gaming, DSL is the better option. Cable and Fiber share their bandwith from people houses to the ISP before they get routed onto the internet backbackbone. Therefore cable and fiber perform differently in different residential areas/cities etc etc.
DSL However does not share it's bandwith from your house to the ISP with anyone else. If you go DSL you have the whole 1-10mbps (depending on the service you have) all to yourself and limited or no hops from your house to the ISP resulting in better pings.
E.g. DSL With Nagles algorithm turned off I can ping Montreal with a 35ms response. Cable is over 120ms with nagles algorithm off. I know guys on Fios with a ping over 400.
Short Story,
Higher Bandwidth != better latency
Furthermore, compared to countries like Japan (tokyo etc), South Korea, etc, the United States internet is pretty far behind when it comes to internet access. In other countries people are pipping 250mbps connections as the norm and there are still many square miles in the US with nothing more than 128kbps dsl or Dial Up. And in just a small in the middle of nowhere town like I live in you can be on one side of the street and have DSL/Cable/Fiber available and on the other side of the street with nothing but dial up available because the companies won't run a line across the street for one house.
We also have right of way conflicts with wire runs and poles etc. E.g. in Shenandoah County virginia all you can get is Shentel DSL, they own all the rights. I called comcast and they basically told me "We'll run it out there, and you'll have to pay install fees, but when shentel services the area and sees are stuff they'll rip it out and you'll have to pay us to come rerun it."
What did I do to solve this problem? Something drastic, I started looking for a new house. My number 1 criteria to my realtor was (access to comcast cable/internet and verizon dsl), I turned down 900 of 1000 homes due to no internet access. Then went through those 100 till we found us the right house and we moved.