There are a variety of reasons why:
1. Too far from an equipped Central Office. This roughly means greater than 4.5km on AWG26 wire, give or take a kilometer depending on line quality.
2. Phoneline is "loaded" with a loading coil added for voice quality. Loading coils filters out all high frequency signals, the very ones ADSL needs for transmitting.
3. Phoneline has AML. AML allows two phone numbers on one single wire pair. Unfortunately, it doesn't work with DSL, and most telcos will not remove AMLs even if you request it.
4. Line has an incompatible service. This can be:
- pay phone lines
- Hotel/Motel lines. However, Canadian hotel owners can get DSL connectivity with DataValet by TravelNet.ca .
- digital lines (FX/RX, Centrex, Microlink, Megalink, DEA, DNA, LDDS). Some DSL providers use analog technology.
- PBX trunks and phone extensions
- CLEC loops (i.e. Bell Canada can't legally put Bell DSL on a Telus loop and vice versa)
Also read About DSL for lots more information