6to4 provides a standard way to tunnel IPv6 traffic at the perimeter over an IPv4 backbone, essentially assigning a range of IPv6 subnets to every public IPv4 address, but it doesn't provide full backward compatibility, and it still depends on proxies at the border of the IPv4 network. The applications at the endpoints need to support IPv6—so all the existing software still needs to be updated—and you still need new protocols for address assignment (SLAAC or DHCPv6) and domain resolution (AAAA records), along with any other protocols than embed IP addresses. Most of this is an inevitable consequence of extending the address space; no amount of clever tricks can make IPv4-only applications, hosts, or protocols automatically compatible with larger IP addresses.

6rd works on similar principles, but in a way that plays more nicely with ISP routing and traffic management policies. The main user-visible change is that a variable-length, ISP-specific prefix is used instead of 2002::/16.

There are a few other transition mechanisms and embeddings of the IPv4 address space, for example the ::a.b.c.d address format. They all suffer from the fact that two-way communication without a proxy requires each endpoint to be capable of representing the full address of its peer. For a node which does not have a public IPv4 address to communicate with a node which does not understand larger addresses, some third node must exist in between to perform protocol translation.

6to4 could've been released to billions of users, with no cooperation from the ISPs, just by home router manufacturers adding a few lines of code (maybe a few dozen, for health checking.)

Applications would still need to change, but apps change a lot faster than ISPs.

6to4 died because it was never the primary mode of addressing. Performance between 2002::/16 and the rest of IPv6 relied on anycast, which was unpredictable and therefore useless for real traffic.

Placing the 6to4 endpoint at the home router assumes that the home router has a public, routable IPv4 address, which doesn't really do anything to solve the problem of address exhaustion. At best it might have been a slightly superior form of NAT, but not enough of an improvement to offset the reliability and performance issues or the lack of economic incentive for ISPs (or anyone, really) to operate 6to4 anycast gateways and potentially invite non-customer, and thus non-paying, traffic to route through their network. Considered as a transition mechanism, the fact that 6to4 works better for two 6to4 LANs communicating over IPv4 (where packets can be routed directly) than between 6to4 and native IPv6 (which requires anycast proxies) was a real deal-breaker. It encourages a dependency on tunnelling via IPv4 rather than a transition to a native IPv6 internet.

6rd, fortunately, has none of these drawbacks, though it does require support from the ISPs.