I admit to only having watched scattered episodes and the pilot, but assuming that the alicorn transformation can only be granted to ponies one could argue upraising the entire population would have a negative effect on the other sapient species such as Gryphons. If she held that position she could even support transponyism inherently, but oppose immediate application until a parallel can be developed for other sapients.

Some possibilities: 1. Comparing their own capabilities and lifespans to that of alicorns could demoralise the other races. 2. While the danger of alicorns to ponies could be mitigated by general upraising or internal policing, the danger to other races could not without the implementation of alicorn rule. 3. As a combination of the above, an alicorn ruling class is innevitable and such as situation might produce individuals Celestia would not trust with alicorn powers. 4. If Celestia upraised trustworthy individuals, she would have to be sure to only select those who would be willing and able to only select others also willing and able to select trustworthy individuals to upraise. This extends to your closest friends and your own children. Upraising individuals on a large scale might lead to a deterioration of the sort of society she is cultivating to maximise happiness and safely make the transition to immortality and greater power at a later date.