There are a couple reasons, to be simplistic.
1) DVB-S2 8psk streams have twice as many bits of data per quadrature as QPSK streams. That's more bits that have to be lined up in the correct order to make a valid transmission.
2) DVB-S2 has higher FEC rates available to it, compared to DVB-S, and the high FEC rates provide less error correction than the lower FEC rates.
Both issues can be handled nicely by larger, well aimed dishes. The Programming providers, for the most part, are not broadcasting for the FTA community. Their intentions are commercial, and they'd just as soon we didn't listen in.
Higher FEC rates are perfectly fine for transmission when the designed link margin is adequately met by a properly sized and aligned dish. These high FEC signals are for commercial use and aren't designed for consumers to receive. They use higher FEC rates so they can get more picture and audio information into the signal instead of having to use more of the data for error correcting.
Because of that, the receiving station design would call for a much bigger dish to be used than most of us have. Most hobbyists are using consumer type dishes that don't meet commercial specs. Using good quality commercial equipment or well designed consumer equipment along with proper tuning and alignment these signals can be received quite well.
Prices are coming down steadily on DVB-S2. I paid way too much for a Pansat 9200 back in March. It was advertised as including the card installed, but the card was the old N2 card and was good for nothing. By the time I paid $200 for the receiver and $135 more for the DVB-S2 card I had almost $350 invested in it. But its working fine and pulling in the signals.
For about $100 you can get a computer card to install in your computer which will bring in DVB-S2. Also, free standing receivers are getting down to the $150-$200 range. There seem to be lots of variations of fta receivers. Some look like external hard drives instead of computer cards. Those plug into your computer cpu. The competition seems to be great.
Check out flea-bay and some of the other web sites!