Why iOS 17 not available for iPhone X?

The iPhone X does not have the hardware required to be compatible with iOS 17.

A quick scan of MacTracker's database indicates that the iPhone X has an A11 Bionic processor with 3 GB of RAM, and only 2 Neural Engine cores. All of the iPhones that can run iOS 17 have newer processors with 8 or 16 Neural Engine cores, and all but two of those iPhones have 4 to 8 GB of RAM.

This suggests some reasons why the iPhone X does not support iOS 17, though you would need to ask Apple (and hope they answered) if you wanted a definitive answer to the question of "Why?"

A further note.

When the iPhone X came out in November 2017, it was running iOS 11.0.1. It can run the latest release of iOS 16, meaning that Apple has provided five major version updates. If you need to run iOS 17 or 18, it might be time for you to replace that nearly seven year old iPhone with a more recent iPhone.