The other factor with turbo pumps in open cycle designs is you end up burning a fair amount of your fuel and oxidizer in them, and throwing it overboard at fair

The other factor with turbo pumps in open cycle designs is you end up burning a fair amount of your fuel and oxidizer in them, and throwing it overboard at fairly low pressures/velocities, which hurts your specific impulse. Staged combustion motors run the exhaust of the turbo-pump into the main combustion chamber, and have far better specific impulse, but are much harder to design and build. The the electric pump design, you can have the simplicity of and reliability of the open cycle main engine, with the efficiency inherent in not effectively throwing a percentage of your fuel overboard.