Because blue actually as the "spell color" was also a "punish creatures and permanents" color in the beginning and nothing back then spoke against using life loss/damage as a means of punishment in blue.
Blue could "strain" the connection between a player and their permanent, either by making keeping it around painful/costly or by making it something not worth risking (as in this case). This style of play has quickly shifted to black and red, but some steps of that shift seem more recent.
Always remember that Blue used to get direct damage in the form of pinging as recently as Seventh Edition, too.
Why is Creature Bond blue?
Because blue actually as the "spell color" was also a "punish creatures and permanents" color in the beginning and nothing back then spoke against using life loss/damage as a means of punishment in blue.
Blue could "strain" the connection between a player and their permanent, either by making keeping it around painful/costly or by making it something not worth risking (as in this case). This style of play has quickly shifted to black and red, but some steps of that shift seem more recent.
Always remember that Blue used to get direct damage in the form of pinging as recently as Seventh Edition, too.