Why is the GS system with the VHA capped at step 10?

All positions within the General Schedule have 10 steps, not just the VHA*.

The short answer is that the position you are is in rated as a GS-6 position (I assume), and unless that position undergoes a review and is changed to a higher graded position due to the demands/requirements of the required standards, it will remain as a GS-6 position.

"Rolling over" to the next higher grade when the step increase is maxed isn't an option because it doesn't work like that. You have to be in a higher graded position to be paid as such. "Why?" is along the lines of asking why a military service member who has maxed out the pay for their grade doesn't get paid as a higher grade with less time. Because they aren't.

Organizations are authorized to give performance-based cash bonuses based on evaluations (up to 10% for "fully successful" or up to 20% for "exceptional"), but many times the employee's management/leader is unfamiliar with the process (assuming they would agree that the employee deserves a cash bonus).

You might want to have a discussion with your supervisor during your performance appraisal meeting. Express that you want to earn that bonus and find out what they require from you in order to do so.
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* General Schedule Overview - https://www.opm.gov/policy-data-oversight/pay-leave/pay-systems/general-schedule/

The GS system only has ten steps for each grade.

GS-7 roles are a higher level of complexity than GS-6 roles. If your position didn't become more complex, regardless of how long you've served in it, the position will remain a GS-6 role and your pay will be capped at that level once you hit Step 10. If you don't want to move out of your current role to take on greater complexity and authority, you will not have earned the opportunity to move into the next higher pay grade. Additionally, step increases are not a "step up" (from a career perspective). When an E-6 goes over ten years for pay purposes, they're still an E-6. That pay increase is not a "step up"; it's just a financial acknowledgement of their longevity.

I've seen this in hundreds of civilian employees over the years. They get a job they like (and are often very good at it), yet they don't want to move into a higher job and/or move to a different organization to advance their career. They want to stay put, yet they still want to get promoted for doing the same job. No organization works like that. Even at a place like McDonald's, if someone enjoys cooking the french fries and don't want to do anything else, they will never become a manager and they will never be paid as a manager.