Understanding the Low Adoption Rate of UID 2.0 in Programmatic Ads

It has come to my attention that the use of UID 2.0 outside of Trade Desk is quite limited. Reports suggest that UID 2.0 impressions make up less than 10% of the total impressions in a DSP transaction. Does anyone know why the adoption rate of UID 2.0 is so low? Additionally, what percentage of Trade Desk impressions are currently utilizing UID 2.0?