The only reason I can think of is because they want more unique identities. More unique people means a greater chance for a purchase. More mail accounts just cost more.

The entire business model of free accounts requires someone paying for something extra. By unique identifying people they can limit new accounts and increase their chances of an upsale.

What if they changed how they operated. Instead of looking for more unique identities why not accept multiple addresses and include an ad at the end of every free email letting the receiver know this came from protonmail. That would give a benefit for each email sent and provide more advertising and give users a reason to upsell?

My guess is having that ad after every mail would bother you (the customer) more than having your identity uncovered.