Perhaps that’s part of it, and cards are more expensive these days so the potential market for people buying two of a new card is even smaller than it was before.
I believe mostly the problem is it’s on the developers to put in the effort to get it to work properly and most developers aren’t interested in putting in the required effort for such a small subset of their market… so developers largely didn’t bother, as a result even fewer bothered getting SLI as most games don’t benefit much, and thus it ultimately becomes a self fulfilling prophecy.
I had 2x 980Ti back in 2015ish and it was really such a waste of money in the end. Haven’t bothered with it since and now like I say it’s not really even a realistic option on Ampere. Next big frontier is MCM GPUs anyway!