Ok, here goes...
1) A HD channel takes typically 4 to 6 times the bandwidth of a SD channel. So changing all channels to HD would require far more bandwidth, even if the SD versions were dropped. That either means losing a load of channels, or introducing new expensive technologies (e.g. switched video; motorised dishes and more satellites)
2) The fact that many TVs are SD-only isn't a problem if they have an HD STB that can downscale. However, LOTS of people still have SD-only STBs. Are you willing to pay for new STBs for them?
3) HD production and broacasting is more expensive, and requires new equipment. Are you willing to pay for all the minor broadcasters to upgrade?
4) Interest in HD simply isn't that great. Although 73% of the UK population have a HD-ready TV, only 49% actually have a HD source (
source).
And many of those who could watch HD, don't. As
an example only 5.7% of ITV viewing last week was HD.