the reason is very simple which are to prevent their citizen from a fraud attempt and to keep their investor interest.
do not you think it's funny to see your citizen and investor suffer due to an unknown investment ?
that is why some countries banned the ICOs and i will do the same because most of them did not deliver what they promised to us
That's the stated reason, but I highly doubt it's to protect the citizens. ICO's take control away from government agencies like the US's Security & Exchange Commission (SEC.) The SEC sets all the regulations for IPO's and alos charge registration and license fees. Then along comes a group of people who launch an ICO and bypasses their regulations completely.
Secondly, ICO's allow the common person to invest in projects whereas you have to meet a high income criteria to invest in venture capital. Again this is government agencies losing power.