I know someone is gonna prove me wrong here, but I'm guessing when they had the idea to do this in the 1st place they obviously had to pick a time and stick to it-2.am is a good a time as any isn't it?
you change back and forward at the same time. If you used 1 am, going back would take you to 12.00, thus raising the potential for legal complications about signatures on forms, birth dates etcet. using 2 am gets it done as soon as poss while keeping you in the same day
They did change it at 1am many years ago but it caused major problems for BT and all their systems - it took days to sort out all the problems it caused with dates etc. so it was decided that 2am was far less problematical.