I suppose some reasons might be:
1. Fuel is a much greater proportion of costs compared with landing fees, airport expenses, etc. on long haul sectors. The LCCs save money on the last two items but everybody pays the same for kerosine.
2. There are already many 'lower cost' long-haul operators: what we used to know as the 'charter' airlines. It might be difficult for a new entrant to significantly undercut them.
3. You wouldn't get much overall saving from short turnaround times.
4. You won't be able to get any greater productivity from your crews as for most 'serious' longhaul they have to get off the plane and go to rest after one sector.
5. It's easy to underestimate the logistics and management required to keep a 24/7 airline running around the world...