Why didn't Baloo and Bagherra, and mother wolf all fight Shere Kahn?
The question is as much puzzling as it is rhetorical. I believe it is because Mowgli vs Shere Kahn was as much symbolic as it was physical. Shere Kahn killed his father and is the king of the jungle, so Mowgli needed to confront Shere Kahn to take its rightful place.
Much like how Luke Skywalker had to confront Darth Vader (or Anakin Skywalker), instead of having Han Solo and the alliance simply bomb Vader's ship.