We have our own issues over here in Sydney with trains that everyone has, sometimes late, cancellations, breakdowns, all that crap..
I'm sure it might be due to some of the older infrastructure in some cities that might no allow it, but I find it weird that many major cities don't adopt the dual level carriages.
There's not really much point, it makes sense for longer distance trips but in many metros and I guess the inner part of Sydney's rail network the line capacity is limited by how quickly you can get people on and off trains. Double deck sort of goes against that, ours have 2 doors per carriage and the RER has 3 but many metros have 4 or 5. Plus the stairs limit movement and accessibility etc... there are a lot of disadvantages. It's something you'd choose if you wanted to prioritise the number of seats over everything else.
That's part of the reason why the new rail line being built is single deck: