It's a good question:  of all the engineers in the world, why choose a Scottish engineer? 

The Gene Roddenberry probably chose a Scottish engineer because of this guy:

That's James Watt, the same guy the unit of energy watt is named after.  He was a Scottish inventor and mechancial engineer who built the first made significant improvements to the steam engine. 

Made sense in the 60's, however given the past hundred years if they were to make a new Star Trek they might have started with a German engineer (or maybe Japanese), but since World War II had ended barely 20 years earlier the 20-somethings that had survived the war were now 40-somethings and seeing a German engineer probably wouldn't have gone over too well.