Because its a first person ego shooter which makes everything just right. You have a HUGE variety of characters: from regular soldiers to zombies, nurses, monkeys, clown, duckmans, burning dudes, robots and many more (evenn bears with funny heats).
You have a HUGE variety of Levels: They are from all the time epochs like the stone age, the second world war, the future. you play inside buildings like hangars or metro stations. outside on beaches and jungles.
You have a HUGE variety of Weapons: not only the random ones like MG's and sniper guns or rocket launchers. you can throw bricks, use electrotools (a weapon which shoots thunderbolts) flamethrowers, harpoons, signal flare guns and many more.
You have a HUGE variety of Game modes: most FPS just give you a deathmatch, team deathmatch and maybe a capture the bag mode. TimeSplitters 1 added Escort (1 man which has to be protected until he reaches his final destination), Last Stand and Knockout. In TS 2 you had 16 different modes to play with each being very individual (Ape assistant, Virus, Zones, Shrink...). TSFP gave you an awesome Base Attack mode were you had to defend or help the preventing the enemy team from completing their tasks.
2.TimeSplitters and the VGAR family?
TimeSplitters was a very well received game with high ratings but hides in the shadow of the more popular FPS like COD or Unreal.
Therefore and because of its unique style and gaming experience, we added it to the VGAR family where games are art. and let me say you this: the TS series are like the Michelangelo of FPS.