Im not aware that 1 year is a hard/fast rule BUT......when you join you are under probabation for 1 year.
And that time period allows you to complete....
a. initial line check
b. 2nd line check
c. all your SO sims (6) including 2 x Aircraft Ratings and a 2nd Instrument Rating
In short, it gives the powers that be enough info to decide whether or not youre a good training risk.....after all training costs a lot of $$$.
Like it or not it does seem logical.