Our daughter Nicola & other half have a beautiful two year old Greyhound called Sally & I was surprised to find that she doesn't automatically, or naturally sit down like other dogs. She seems to manouvre in an awward way before sitting down.
Apparently their back legs are so long & their big muscles restrict their ability to do so as easily as other dogs do.
Having said that, there may be an expert on here with another explanation.
Purely the fact that their back legs are so long and angulated it is difficult but not impossible. Its easier for them to lie down than sit. I did once see one doing obedience though and it sat on command.