Hi,
here' the idea, simply put.
A 'zombie' is a dead person walking around.
Some companies are like zombies, because they are dead in the sense that they have lost their success and vitality, but they are still walking around in the sense that they are still in business.
If you save such companies, you are making it more difficult for newer and more vital, more lively and energetic companies, to achieve success.
Clive