From the IP9 Users Guide:
Most ImagePrint profiles are done for 1440 (Photo) droplet sizes so often only 1440
Quality settings will be listed. This is because this mode typically has the advantage of variable size--the size of the ink droplets can vary througout the print. While smaller in size, the 2880 (Best) mode is a fixed droplet size which reduces some of the advantage a variable droplet can add in producing a smooth screen (dither) pattern.
For most papers (excluding very high gloss papers) most users find no benefit in using the finer, fixed droplet sizes and prefer the significantly faster 1440 variable droplets. This is especially true on papers with a high dot spread such as rag and other matte papers.
If you download a number of matt and glossy paper profiles, you should see the ability to select 2880 dpi, but this depends on the paper as stated above.