Jerome said:
Because 32mm was the smallest distance posible in economic terms to have counter rotating line boring machines built when the system was developed. Its importance is that it became the standard. If the spindles of the multi-head hole boring equipment of the day could have been set to 30mm then that would have become the standard.
Line boring machines weren't created in the stone age. The only thing limiting spindle spacing is the diameter of the gears. They could even get a lot closer than 30 mm.
Making a gear with a 15 mm radius (30 mm diameter) is pretty easy to do. It doesn't even take a lot of calculating (especially with a whole number gear module). You can use a metric gear module of 1 and 30 teeth, or a gear module of 2 and 15 teeth. Either way you end up with a metric gear with a 30 mm diameter. The 15 mm radius means two gears, when mated, would be 30 mm apart. It didn't even require fractional/decimal gear module values.
You mentioned "economically". That could very well be true. It could be that a 1.25" gear was more common and therefore cheaper than a 30 mm gear. But then that was exactly the point I was making previously. The same situation occurs with chains (except in reverse). A metric chain is cheaper than a near identical imperial chain because metric chains are more readily available.