Because Obi Wan was a brother to Anakin when he needed a father. That's what makes Qui Gons death so much more devestating, as it sealed the ultimate downfall of the Jedi and the Republic as a whole
In fact, while it would have been more difficult to make it a neat and tidy film, it would have been great to have a bit of development time and a flash forward for at least a few months (though a year or two would've been better) to show Qui-gon with his two apprentices, to the consternation of the Council, and having things going great for the three of them, bonding like a sort of family as it were.
Then Qui-gon is killed, Obi-wan is driven by his attachment to both Qui-gon and Anakin to step into the mentor role before he's ready, to preserve what's left of this attachment dynamic, and things go downhill from there.