Yes there was a reason, and that reason was NOT because it was ineffective. Sorry to burst your bubble on that. The reason was because traditional SLI worked on the driver level, and it took a lot of extra resources on the part of Nvidia to build and maintain SLI profiles for every game.
Around the time that DirectX 12 came out, there was a push in the industry for games to get “closer to metal”. That phrase refers to diminishing the role of a graphics API (Such as DirectX) as a translation layer between the hardware and the game, trying instead to increase opportunities for a game to be able to communicate more directly with the underlying hardware. This was already common with game consoles, and is why many games are able to squeeze extra performance out of the relatively modest hardware in most consoles. The idea was that maybe this could be done with computer games also.
So following that theme, Multi-GPU support in DirectX 12 did not go away, but was instead shifted into something that game developers would need to program directly into their games. Nvidia was more than happy to relinquish that responsibility. When game developers actually do this, it can result in better Multi-GPU utilization compared to traditional SLI/Crossfire, and can even allow for interesting configurations such as using mismatched cards. But while that was great in theory, the reality is that most game developers did not care enough to take on that extra effort for what represented a fairly tiny percentage of their customers. Nvidia had incentive to make SLI profiles because it helped sell more cards, but there is no similar incentive for game developers. DirectX 12 multi-GPU was not “abandoned”, it never really existed in the first place (due to lack of support from game developers). DirectX 11 SLI continued to be supported with new SLI profiles until most new games began to be released as DirectX 12 only.
But for those still using older hardware, still using DirectX 11, and playing a game that still has an SLI profile (such as WoW), it still works great, even to this day.