The Cybermen have always been (to my understanding) humans who replaced their bodies with robotic/cybernetic parts which is pretty much what RTD did in writing the backstory for the Cybermen.
In the "Ultimate Guide" RTD is described as a 'superfan' and is quoted as saying that he wasn't looking for a 'reboot' but describes his effort in 2005 as a 'clean start'. He said he did not discard the history and canon of the show but needed to reintroduce old villians to a new audience, ie. The Master ("Utopia" etc), the Daleks ('Dalek'), the Macra ("Gridlock") and the Cybermen ('Age of Steel').
Also to be taken into consideration is the vast difference in filming technology and special effects between the show's debut in 1963 and its return in 2005.
Of the three main enemies (the ones most associated with DW) the Daleks, the Master, and the Cybermen, there was no real set origin cannon for the Cybermen established other than vague references to Mondas, Telos, etc. As there was for the Daleks (Mark 3 Travel machines developed by Davros, mutated survivors of nuclear war, etc. Ref: 'Genesis Of The Daleks') or the Master. RTD took advantage of the opportunity to create a backstory for the Cybermen.
Simple answer? Because men wrapped in tinfoil do not equate to believable enemies in this day and age of HD media, computer graphics and overall budget changes. They had to be reimagined to give fans a good reason for the redesign while allowing for the same enemy to continue to appear in the new series. A similar approach was made with the Daleks in Series 5.