The reason this is popular is that if you do your CFII first, then go back and do the CFI, you don't have to teach the BAI skills in CFI, since you've demonstrated that previously in CFII.

If you do it the other way around, you have to teach BAI for CFI and then essentially again for CFII. It's notionally cheaper to do it CFII first, and for 141 schools competing with each other neck and neck, this is sometimes their chosen route to do CFII and CFI. So many other things will throw a wrench in or affect the total cost for a certificate/rating, so I don't generally recommend this for the cost factor, but it remains popular.