In a GDPR world, any company that handles data from the EU or from users originating from countries where GDPR applies must display unambiguous consent notices to its users for every single partner it works with. This is logistically and technically problematic for publishers. User experience can be jeopardized by bombarding them with multiple consent notices, at the risk of driving them away from using an app or website permanently.
More details about GDPR and how publishers should align can be found in the Introduction to GDPR section.