Idk if I have the formulation right on this but I imagine it’s a combination of various things:

• Conservative nostalgia, concretely for fries themselves and abstractly for a fake, idealised version of the past.
• Crackpot health woo: beef tallow is “carnivore” and “caveman” diet-coded. It’s not a processed/GMO seed oil.
• Conspiracy by US Big Dairy to beef up their numbers.

I can’t speak for the taste. However, the fat used to fry fries does make a difference, as fat is a conveyance of flavour. Many fry recipes call for peanut oil, for example, while many people prefer using a neutral oil like canola to deep fry. I don’t think I know instinctively what the taste of beef tallow is like, but I imagine it’s just… beefy?

Notably different oils have different smoke points, meaning you can fry at higher temperatures for different textures. That’s why duck fat potatoes are a thing, for example.

Malcolm Gladwell famously has a podcast episode on the crusade against beef tallow, where he also replicates mcdonalds fries with beef tallow. But Gladwell is a chud, so fuck all that.

People remember fries used to taste better, and they'll attribute it to anything except the maturation of their pallate.