Modems have several problems.
First, they're slow. It's unlikely that modems will ever provide much more throughput because of the relationship between frequency, distance, and signal strength. Current FCC limits and phone company standards allow only a certain frequency range to be transmitted at a given signal strength or voltage. This restricts POTS lines to the current top throughput because hardware vendors just can't squeeze any more information within those proscribed limits. ADSL, as we'll see, breaks out of those bounds entirely.