I think because it is the most ethical/repairable phone you can buy (as far as I know). Now days a lot of phones are very similar to each other, with slighy different specs. Being repairable makes the Fairphone standout, which means it will get more clicks than another boring android.

Also, the right to repair movement has gotten quiet big (at least among tech enthusiasts) recently so products like the Fairphone and Framework are more popular becase they are exactly what this movement has been calling out for.