Bit of an aside, but the "horizontal" propagation of laws from state to state has some structural reasons that it favors certain kinds of laws more than others. Mostly the large role of the Uniform Law Commission (ULC), which draws up model laws and organizes campaigns for them to be passed: https://en.wikipedia.org/wiki/Uniform_Law_Commission. It has no formal power, but it has a well-established set of supporters, working relationships with legislators, staff to do the actual drafting work who are familiar with the law of all 50 states, etc.

The ULC in principle is interested in a lot of areas of harmonization of what they consider well-written laws. But its biggest political support is from multi-state companies that want a uniform commercial operating environment, so it has been mostly successful there, via the Uniform Commercial Code (UCC), a set of model regulations for commerce. Businesses can get some of what they want in this respect more directly, by lobbying the federal government to directly pass uniform national laws, enforced via federal preemption. But it's possible to achieve a more comprehensive degree of uniformity, avoiding constitutional issues with federalism, by going the UCC route. It formally preserves federalism (each state voluntarily enacts the UCC), while still producing the desired outcome due to the heavy pressure states feel to conform to the UCC, lest they be perceived as "bad for business" (every state has passed it, though in a few cases with modifications).

The other area they've been somewhat successful in is family law, due to widespread concern over problems that arise in cross-state divorce and child-custody proceedings if the states have very different family-law regimes. So many states are willing, absent some strong lobby in another direction, to sign on to a "national best practice" in that area.