Why don't you see subway trains with diesel locomotives?

www.railway-technology.com

I'm not sure whether I took up this metro long time ago. But, I'd like to take up here.
Its metro system is built by a rail company in China. It doesn't seem to be strange, considering current examples of the railways in Africa.
But, if you see the picture on the website, you will notice that metro train is hauled by diesel locomotive. I've never heard a railway built as a metro line has trains running by diesel or steam engine, except for London Tube, which initially operated steam locomotives before commercial use of electric trains.
After electrification, diesel locomotives will be removed. So, it's time to experience diesel-powered metro.