The closure – proposed at little over a month's notice – has been vigorously opposed by local businesses and residents.
A TfL spokesman said it was impossible to keep the station open by replacing the escalators one at a time.
"The escalators at Shepherd's Bush (Central line) station have not been replaced since they were built in 1920s," he said.
"Installing new escalators is a large and complex project which will require the concrete foundations and steel framework to be broken out and rebuilt.
"The two escalators are in a single small shaft next to one another, with the escalator mechanisms very close together, so we can't demolish and renew one escalator and keep the other escalator working."
Residents have demanded an explanation for why escalator replacements in other Underground stations have not needed station closures.
TfL has responded by saying that other Underground stations have a fixed stair between the escalators and more room in the escalator shaft.
"Without at least one escalator in service we can't keep this deep level station open with just a spiral staircase for safety reasons," added the spokesman.
The replacement works are being project-managed by developer Westfield, which is building a nearby shopping centre.
It is understood to have contributed around £170M to public transport upgrade works currently taking place at the nearby
Shepherd's Bush Hammersmith & City Line station, to a heavy rail station and to Wood Lane and Shepherd's Bush Central Line stations.
Local opposition to the station closure is being led by Ealing, Acton & Shepherd's Bush Labour MP Andy Slaughter.