Only because Australia does have its own uniform postage, and only because it took over a decade to achieve, is the intriguing questioned asked Why. Well, there hasn't been any good reason for it to have occurred in the first place. However, because it did occur, and took so long, the question needs answering.
The Australian States period is also known as the book-keeping period. This, due to a clause in the Constitution which stated that 75% of all profits from departments previously under the control of individual colonies, were to be returned to those colonies for a period of up to five years after federation. In fact, this period was later extended for another five years to eventually cover 1901 to 1910. The reason it was extended, was because the system worked well!
The Federal Postmaster Generals Department was established on March 1, 1901, two months after federation, and inherited the existing postal rates of the colonial administrations. The bottom line was that each state had differing rates of postage, and, naturally, fought against any attempt at uniformity because it would severely affect their revenue. This made it impractical to implement a uniform colour scheme, or a single design, for a given denomination (either scheme could be used for identification of the purpose of that rate). A 2d red would be required in South Australia, a 1d red in Victoria, and a 1d, or 2d red in other states, depending on distance travelled!
Secondly, it was thought to be difficult to keep track of individual State postal revenues if all states used the same stamps. This because, in those times, it was common practice to exchange money via stamps, and additionally, self addressed envelopes and their like would distort the figures. Whether these fears were justified is moot, it was just plain easier to maintain separate postal administrations under control of the commonwealth.