It's because of the degree structure at Scottish unis.
At St Andrews, if you don't take further maths, you have to take the 4-year degree so in your first year, you'll take other modules in subjects that aren't maths (only 1/3 of my 1st year was maths), and in second year, you are strongly encouraged to take modules in another subject (I didn't).
However, if you take further maths and have good enough grades, you can apply for 2nd-year direct entry where you will focus on maths from the very beginning. Edinburgh will be similar to this.