Lionel made a decision some years back, likely related to the high costs of new tooling, and the very low volume of tinplate sales, to license their name so that MTH could use it, and hopefully spur their sales of tinplate, with Lionel taking their cut. I'd guess the idea was that these iconic toys would sell better with the Lionel name on them than with the MTH name, particularly for the reproductions. The plan was perhaps that MTH sales would increase, and Lionel would make some money with essentially no effort/expense.
Whether that has worked out as hoped, or not, who knows? But the licensing agreement has now been renewed once, so both wish to continue with this arrangement after a few years of trying it out in the marketplace. On the other hand, MTH started selling tinplate on their website direct, something that does not speak to resoundingly successful dealer network sales of these niche products. One way or the other, we're lucky that any tinplate is being made, much less the variety offered by MTH, which is substantially greater than anything available in new product in the last 60+ years.