This is simply because it is among the most acidic of beverages on sale. It has a pH of 2.8. Some vinegars are less acidic than that (Wikipedia reports taht cider vinegar has a pH of over 4, which may explain why my chilli preserve with cider vinegar went mouldy) though strong vinegars are down to about 2.2. Lemon juice is usually slightly more acidic than vinegar, around 2.0. But I expect that coca cola is generally rather cheaper than vinegar or lemon juice. Maybe if you could bulk source vinegar it might be cheaper.

The potential trouble with using coca cola is the sugar in it which would make your drive sticky until it had been washed by the rain, and you wouldn't want to be treating it except when you could get a couple of dry days to allow it to get to work.

I use Patio Magic, which contains benzalkonium chloride. A 5litre container retails for around £20, but you dilute it 2 or 3 times so get about 10-15 litres to apply. They say that treats around 170 sq m, but my drive is 140 sq m and I don't quite get coverage of it all, so I concentrate on the worst bits. Smaller containers also available. I suppose that is still a bit more expensive than coca cola. I was surprised at the admonition that the moss and algae gradually reduce after the initial kill, but it seems to be true, the drive continues getting cleaner for several months after application. I wait until spring to apply it.

There is a product called Algon, which claims to be "safer" and "organic" but they won't say what the active ingredient is. I wonder if it is just a strong vinegar.