Because Tomb Raider was one of the first of its kind and set the standard for realistic 3D plaftorming, after Prince of Persia 3D fucked it up. Ironic, because PoP improved itself on platforming starting with the Sands of Time trilogy. That said, I never played any Tomb Raider after the original trilogy. I was not exactly a big fan of the games, and any news ones that came afterward I responded with mostly a meh. The fact that there better female characters before, during, and after her time did not help matters. Before anyone starts: Aya Brea, Claire Redfield, Jill Valentine, Sheva Alomar, Sherry Birkin, Tyris Flare, Blaze Fielding, Lady (DMC3), Chun-Li, Elena (SF III), Maogichi Saika (Sengoku Basara), and Bayonetta.
The new games aren't much better, because it literally copies Uncharted and throws it in to a open world. Lara never develops as a character, the plot is more convoluted than any of the old-school games, and all three games themselves aren't all that challenging compared to the older titles. For Christ sake, Lara is still becoming the Tomb Raider. Say what you will about the older series, but at least she was already a tomb raider since the first game.