Flash TV Show, if Reverse Flash has never been born, than why is Barry's mom still dead?

Because the theory The Flash uses(which is very close to a real-life theory on time-travel by the way, it's a very interesting part of theoretical physics, and most good sci-fi uses parts of these theories, so sci-fi might even be more educational than you'd think) says that time works in Events, and that even though Reverse Flash never was, the events he was part of, might be slightly altered, but most of them will still happen, history wants to self-correct.

So where some events have changed, history will try very hard to push someone else into Reverse Flash's role, to make sure history is still the same.