BA Is for British people, who include Scottish, Welsh and NI. . . Scotland is still apart of the Uk, so it does have a national airline, which is BA.
Don't forget Scotland only has a total population of a little over 6 million people. More people live in London alone.
Back to the forum, Ryanair does tend to fly to sun routes from PIK, but at the end of the day, it will only fly where the demand is.