Because Meteor front-end bindings suck terribly. With sails you can use any of the tried and true front end frameworks like angularjs or emberjs etc instead of meteor's crap.
Disclaimer I haven't used Sails so this is not an endorsement for sails, just a reflection of my experience with meteor.
Disclaimer I haven't used Sails so this is not an endorsement for sails, just a reflection of my experience with meteor.