UEFA chose to move the main event to Portugal because the country is one of the few countries that the UK can easily travel to under Covid-19 restrictions.
On the other hand, Turkey is on the UK travel red list, which means that people are actively discouraged from visiting the country.
The European Union stated that the British government has always been willing to hold the finals in England to avoid unnecessary travel by fans, but could not reach an agreement on the necessary exemptions from the British quarantine arrangements.
The final will be held at Dragao Stadium on May 29th. UEFA stated that 6,000 fans from each club can participate.