Well, Sujod/prostration wasn't Haram/prohibited for previous nations. But it was allowed as glorifying not as an act of worship. You can read
ورفع ابويه على العرش وخروا له سجدا وقال يا ابت هذا تاويل رؤياي من قبل قد جعلها ربي حقا وقد احسن بي اذ اخرجني من السجن وجاء بكم من البدو من بعد ان نزغ الشيطان بيني وبين اخوتي ان ربي لطيف لما يشاء انه هو العليم الحكيم
And he raised his parents upon the throne, and they bowed to him in
prostration. And he said, "O my father, this is the explanation of my
vision of before. My Lord has made it reality. And He was certainly
good to me when He took me out of prison and brought you [here] from
bedouin life after Satan had induced [estrangement] between me and my
brothers. Indeed, my Lord is Subtle in what He wills. Indeed, it is He
who is the Knowing, the Wise.
Surat Yusuf - Verse 100
As of why, I think we shouldn't bother asking Why Allah -SW- did this or that, rather we should submit to Allah's orders. But we can say that Sujod/prostration to Adam -AS- was because Allah -SW- wanted to show Adam's place to everyone.
If Angels made Sujod/prostration to Adam, then Humans/Sons of Adam shouldn't lower there place and follow Shaitan. And Allah knows best.