Archer entered Spirit Mode because his servant core(the heart) is not fatally damaged or destroyed. Archer is a low Mana requirement servant, so he can survive for a bit longer. That is the official reason given by the author in Q&A sessions.
Even in Fate/Zero she cannot enter spirit form.

You have to read [Fate] route (Saber heroine route) in the Visual Novel since it explains Saber's background and story as well as her motivations to join the war. This anime is [UBW] route (Rin route) which does not explain Saber stuff.

Any further explanation will be a spoiler for Saber's background story. If you have read the VN but forgot the reason or don't want to read an 11 year old novel then click the spoiler box below.



This post has been edited by FatalExe: Jun 22 2015, 12:59 AM