But they never forgot the mother Rhoyne and what forced them flee from their mother. Rhoynar then sought refuge in Dorne and adopted the Dornish identity while native Dornishmen adopted most of Rhoynar customs. Nymeria and her people were forced to fled from their home region of Rhoyne when Dragonlords of Valyria sought to conquer them. House Martell of Dorne: House Nymeros-Martell (or as it is commonly known, Martell) is descended from the Rhoynar Queen Nymeria. There are following reasons in my opinion: Vishvesh has added a very good answer but he has skipped handling the cases of the individual houses you mentioned (Except House Lannister). So unless a house was lucky enough to have had a Valyrian steel as heirloom it is very difficult to get one. And house who have Valyrian steel, do not sell them.The little lordlings would gladly part with their daughters should a Lannister come asking, but they cherished their old family swords. Thrice at least Lord Tywin had offered to buy Valyrian longswords from impoverished lesser houses, but his advances had always been firmly rebuffed. He had never returned nor had Uncle Gery, the youngest and most reckless of his fatherвЂ™s brothers, who had gone seeking after the lost sword some eight years past. The old Kings of the Rock had owned such a weapon, but the greatsword Brightroar had been lost when the second King Tommen carried it back to Valyria on his foolвЂ™s quest. It had always irked his father that none belonged to House Lannister. Only the best weapon smiths can reforge swords from existing Valyrian steel.ДЄt long last, Father? Valyrian steel blades were scarce and costly, yet thousands remained in the world, perhaps two hundred in the Seven Kingdoms alone.New Valyrian steel could not be created.We can make following deductions from the above statment: Only the greatest weaponsmiths can reforge swords from existing Valyrian steel, making those remaining weapons highly treasured and extremely rare. Valyrian steel was always costly, but it became considerably more so when there was no more Valyria, the secret of its making lost with the Doom.

Valyrian steel was manufactured in the Valyrian Freehold with dragonfire before the freehold fell. The exact reason why some house don't have Valyrian steel is not explicitly explained.

Ck2 agot valyrian sword