As Chris pointed out the resourcing system makes this fairly unlikely, but I guess they felt that with one game the chance was higher (even if still fairly low) so why not fill in that time by allowing a mulligan?
Because it allows you to build a combo deck that requires a card or cards to be in your starting hand to work. With the old system, if you didn't get that card, your all-in was screwed and you lost. With the new, you get two chances to get your broken combo card(s) in your draw. So the mulligan rule being added allows for more degenerate deck designs to succeed.
Which is boring.