Why doesn't higher frequency mean higher data rate?
It's not a bad question and shows quite a common misunderstanding of how Radio Frequency (RF) Systems work.
The 900MHz and 2600MHz signals are called Carrier Frequencies. The actual information is contained in the 10MHz bandwidth. The original signal is a baseband signal that extends up to 10MHz. This is used to modulate the carrier signal. The reason we do this is so that we can have several channels sharing the same medium.

When received by a radio, the signal is downconverted back to the baseband (up to 10MHz in this case). The reason we do this, instead of sampling the signal directly, is the RF electronics are very complicated and relatively expensive, while baseband electronics is not.

So to answer your question, when both signals are downconverted, they are both 10MHz wide signals, so will transfer data at that rate (probably greater for QAM due the symbol rate but that's another story).