This is largely due to the Americans with Disabilities Act. Accessibility in the US is mandatory.
While ADA access is important, it is not often the most important thing. But the lawsuits are forcing $100+ million settlements that take any prioritization of resources out of the hands of local voters or local legislatures.
Why spend $300million on sidewalks 'right now' when other services may be more effective (e.g., on demand shuttles, etc.)