Two reasons: 

1) accessibility
2) compliance

read more here: https://www.blink.sv/blog/simplifying-bitcoin-user-friendly-account-creation-and-recovery-in-blink

>  if the authorities request user data, will you provide it? 
yes, Blink is a regulated financial institution and if Salvadoran authorities make a valid request, then Blink will share information with the authorities, and has done so in the past. Blink also must submit SARs (Suspicious Activitiy Reports). 

pretyflaco

Why does your service require a phone number to sign up? And, very honestly, if the authorities request user data, will you provide it? In almost every country, a phone number is linked to a national identity. Please answer this question honestly, if I find it convincing, I’ll start using your wallet right away.

pretyflaco aka Kemal of Blink & Adopting Bitcoin here - AMA!

Why does your service require a phone number to sign up? And, very honestly, if the authorities request user data, will you provide it? In almost every country, a phone number is linked to a national identity. Please answer this question honestly, if I find it convincing, I’ll start using your wallet right away.

