In addition to the other replies, there's also the problem of simply igniting the flame. For example, a Falcon 9 uses kerosene and liquid oxygen, which burn real nice when ignited, but won't do anything if you just mix them together and let them sit. Just like lighting a stove, you need something to get the process going. With a Falcon 9, this is done by injecting a small charge of two chemicals called TEA and TEB. They're hypergolic with each other, meaning they ignite on contact. This then starts the combustion of the actual propellants. The amount of TEA and TEB carried on board is limited, so you can only restart a certain number of times.

Using hypergolic propellants solves this problem. The trouble is that hypergolic propellants are inherently dangerous (they'll explode if they come in contact, that's the whole point) and are typically highly toxic. They're OK in small quantities, thus they show up in RCS systems, but making an entire orbital launcher using hypergolic propellants is a massive pain. It has been done, but the result tends to be more explodey than usual.