The big reason is that domestic ISPs don't want to switch (not just in the US, but everywhere really.)

Data centers and most physical devices made the jump pretty early (I don't recall a time where the VPS providers I used didn't allow for IPv6 and every device I've used has allowed IPv6 in the last 2 decades besides some retro handhelds), but domestic ISPs have been lagging behind. Mobile networks are switching en masse because of them just running into internal limits of IPv4.

Domestic ISPs don't have that pressure; unlike mobile networks (where 1 connection needing an IP = 1 device), they have an extra layer in place (1 connection needing an IP = 1 router and intranet), which significantly reduces that pressure.

The lifespan of domestic ISP provided hardware is also completely unbound by anything resembling a security patch cycle, cost amortization or value depreciation. If an ISP supplies a device, unless it fundamentally breaks to a point where it quite literally doesn't work anymore (basically hardware failure), it's going to be in place forever. It took over 10 years to kill WEP in favor of WPA on consumer grade hardware. To support IPv6, domestic ISP providers need to do a mass product recall for all their ancient tech and they don't want to do that, because there's no real pressure to do it.

IPv6 exists concurrently with IPv4, so it's easier for ISPs to make anyone wanting to host things pay extra for an IPv4 address (externalizing an ever increasing cost on sysadmins as the IP space runs out of addresses) rather than upgrade the underlying tech. The internet default for user facing stuff is still IPv4, not IPv6.

If you want to force IPv6 adoption, major sites basically need to stop routing over IPv4. Let's say Google becomes inaccessible over IPv4 - I guarantee you that within a year, ISPs will suddenly see a much greater shift towards IPv6.