because its generally more accurate, harder to offset.. The thing people need to realise is that someone is booking the trade somewhere at sometime. IT might bounce around 50 people first but eventually someone is taking the opposite side of the trade whether its your broker or another person on the other side of the planet. EA flow, usually trades more often, each trades last less time and the rules are mechanical. So the time to offset can be difficult or it is harder to make profit on the order if its highly accurate.
In FX, the best type of flow is manual flow, reasonable size, that always goes into a drawdown of 5 or more pips (on eurusd for example). That way you can make money, the LP who offsets can make money and everyone is smiling. If you predict entry to the pip every time, it doesn't matter if you are using your hands or an EA, no one will want you because they cant make money on the flow.