Not only because it's easy to type, it's close enough to -> be easily read and understood as an extension for a special case
This follows the same approach like == and === in JS, where repetition indicates a specialization.
~> looks nice at first glance, but is too easy to be overlooked in some fonts and at the moment I have to deal with too many low level Perl users who wouldn't spot it.
?-> or &-> might have a derivable logic, but look too much like line noise.
And it's not very consequent to allow &-> if we are sure that there will never be a |-> or !->.