Why does core use print rather than echo?
echo is only faster in academic environments. The difference is very marginal. A quick benchmark I just did: The time fetching one row from a table with more than 2000 nodes is 1 unit. 

In order to get an improvement of 1 unit, I would need to replace about 3.700.000 (over 3 milion) print statements with echo. In other words, if you want performance improvements, this is hardly the place to start.

Seen that print can be used in a much wider range of cases then echo, the first has some advantages. It leaves you three options:


Always use print
Use echo where possible and fallback on print
Use echo always and refactor your code if echo cannot be used, to use it


Drupal has defaulted on the first.

To answer your actual question as to why this was chosen: It was not. It simply grew this way.