1. First is the issue of reach. With its "shared neighborhood" style of connection, cable companies reach hundreds of people in a local neighborhood with a single pre-qualified line. DSL customers, on the other hand, each require their own line, and the provider incurs extra expense in managing each of these individually.
True, most households already have telephone lines, but these require some form of testing or "qualification" as some lines (especially older ones) might not be of sufficient quality to support data traffic.
2. DSL technology also is distance sensitive. Essentially, the longer one's telephone line runs from their house to the phone company, the less performance they can achieve with DSL compared to neighbors who might live closer to the public exchange. This creates a marketing problem for DSL providers, who must convince customers that the variation in performance isn't a serious or "unfair" limitation.
3. With cable modem, customers find it very convenient that the same company they use for cable TV service provides them Internet access (usually on a single monthly bill). Unless a person signs up for DSL with their local telephone company, DSL customers do not enjoy this same convenience.
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