Mpeg 4 is new technology and most of the people only have MPEG 2 STBs . Many people living in rural areas don't know the difference between MPEG 2 and MPEG 4. Many people are still buying MPEG 2 STBs bc they are cheaper.
So simply MPEG 4 doesn't have that much coverage as MPEG 2.
That's why big brodcasters aren't interested in buying these slots but small brodcasters buy them because of low price.