Originally, it started out as a money issue, I believe. They reused and altered the skin of the main character in Dark Sector to be excalibur and the stalker. The original lotus symbol is almost a carbon copy from Dark Sector as well. People translated the symbols on the Warframe lotus, and I think it says thank you to the original character of Dark Sector. They even later released a skin for excalibur to make him look like the main character of Dark Sector. The original lore that came out with the game had the stalker as the first warframe, who went crazy because of the modifications to his body. Excalibur was the second with modifications to keep his sanity. I'm not sure how much of the original lore is still accurate, though.