I don't entirely agree with the upvoted answer. I have read somewhere (but cannot locate the source right now) that the electrical power transfer pathway is 70% efficient in Toyota Prius, whereas the mechanical power transfer pathway is probably over 90% efficient. Consider this: the electrical components (inverters) have water cooling, but the mechanical power transfer components do not. This illustrates that the electrical components produce more heat than the mechanical components, and are thus less efficient.
It is true that the electrical components would need to be much bigger to have series hybrid operation. In a series-parallel hybrid, part of the power is transferred mechanically, and thus only part needs to be transferred electrically. Therefore the electrical components can be small. But this is not the only reason: efficiency is another major reason.
In fact, if you're building a plug-in hybrid electric vehicle (PHEV), you need to have big electrical components. Then you could in theory have series hybrid operation. But it would not make sense, because PHEVs are occasionally driven long distances, and then the power transfer from the gasoline engine to the wheels needs to be as efficient as possible. Efficient power transfer means mechanical power transfer.