Well we are talking about a game which was released in four parts between 1977 and 1983 - and last printed in 1998 (assuming AH continued printing CoI, CoD, and GI up to the end, which I doubt they did.
When you cut through all the nostalgia BS, OSL is fatally flawed due to its lack of opportunity fire. Using the rules as written, I move all my units in the movement phase, then in the defensive fire phase YOU MOVE MY UNITS BACK and fire at them. I would love to see proponents of OSL try this in a scenario like Hill 621, with 26 squads for the Russians. Totally unplayable and broken IMO using the RAW.
There is an optional rule hidden in the back of the rulesbook which restore playability by having the defender fire at the attacker as the attacker moves. The problem with this is that there is no ROF, SFF, FPF, residual fire, etc - the defender gets one shot and that's it. This restores playability, but without residual fire you can rush as many units as you want through a hex and the defender gets only one shot. Playable but still irretrievably broken IMO.
So what accounts for OSL continued popularity? In one word, actually two, nostalgia and solitaire. There are lots of reclusive gamers who actually enjoy playing solitaire where they don't have to be concerned about social interaction, and they can maintain the fallacy of believing their "perfect plan" is indeed perfect without having to test it, or their actual knowledge of the game in general, against a real opponent. And of course, the few times they have played against a real person, and realized their "perfect plan" was build upon a house of cards, this only makes them more reluctant to play ftf again. For people like this OSL is the perfect game - the chaotic organization of the rules across 4 rulebooks does not matter since when playing solitaire they are the sole interpreter of the rules, the broken defensive fire system does not matter since when playing solitaire there is no real winning player, only a winning side, and the fact that these games have been out of print for over two decades does not matter since there is no need for another person to play the games with anyway.