Because the return part of the mission relies on nine identical engines right now. So if one fails it is not a problem. But if you replace the central one with a different one that becomes critical for the return mission then you have made matters worse.
> Because the return part of the mission relies on nine identical engines right now. So if one fails it is not a problem.
Is that true? Doesn't the landing have to use the center engine - any other single engine would be asymmetric, and multiple engines would have too high a thrust.
Yes, if the center engine fails, the landing fails. There's no way to recover from that.
But the great thing about SpaceX's system is that it starts with a cheap expendable rocket, then makes it recoverable. You lose one? Who cares, build another one. They could have a 50% success rate recovering the first stages and it would still make things amazingly cheaper.
For future reference, the return part is made up of 3 burns. The first two use three engines, and the landing burn uses just the one. Coming back, it's obviously much lighter without all the propellant and 2nd stage/payload weight, so it needs much less thrust to reverse course. See http://flightclub.io for more info.
