Advice about my letter of explanation on program changing for my 3rd try

[DUNNO]

Hi guys,
I've got two rejections for my study permit :'( :'( :'(
And I am changing for a new PGC program in a new school instead of the previous diploma program, and planning to use it for my 3rd application

So, should i explain the reason i changed my program and school in my SOP?
or should i write a specific explanation letter to clarify the reason, and apply it attached with my SOP?

I am just afraid of the SOP pages will be too long.
Did someone have relevant experience? I really need your help!

Thank you !