I don't think it's a question of players not UNDERSTANDING the concepts of defense, at least in a broad sense. I think most players understand that it's easier to fall back and defend one position, and they do tend to "get" the concept of reinforcements, etc. They might not get the more nuanced points of setting up a GOOD defense (IE: overlapping fields of fire, minimal exposure and chance to be flanked, etc.), but overall I think they "get" it.
The problem, as I see it, is purely psychological. People just don't WANT to defend. It's not what they find fun. Some of this has to do with basic impatience. People want to get out there and kill kill kill. Some of it has to do with wanting to sc0r3 t3h p01ntz and thereby be 1337. In that sense, it's more about their PERSONAL experience in the game, rather than their TEAM winning the round. Who cares if the team won as long as I got top scorer, right?
Mostly, I think it's impatience, though. They DO view defending as boring because it requires you to wait for the enemy to come to you. The problem isn't unique to Red Orchestra, either. It's existed in games like BF1942 and the Forgotten Hope mod for YEARS. In that game, often, you only need, say, 4 positions of 5 held to make the other team's reinforcements "bleed out" (basically tick away with the clock). You only need 3 positions of 5 to have no bleed whatsoever, and run out the clock for the map. But people want to gogogo killkillkill and will regularly make suicidal charges to try to retake a single position (thereby depleting their OWN reinforcements) despite the fact that it's basically indefensible.
It's like trying to recapture the farm from the 2nd Line position on Berezina.
And yet, players will do that. Some of this may be that the maps don't make you instantly feel exposed. Compare the approach from the German spawn on Stalingrad Kessel to the Warehouse to the approach form the Russian spawn at the 2nd Line on Berezina to get to the Farm (I don't remember if the farm's recapturable, but you see my point). In the one case, you've got at least some chance of making it to the capture point, and then if you believe you can actually win, you might get the drop on one or two guys and push the enemy team back if there's enough buddies with you.
On Berezina, though, you're charging across open ground, HIGHLY exposed, etc.
This may partially be due to the fact that many of the stock RO and custom infantry maps are relatively small scale. Berezina and Gorlitz are large-scale combined arms maps with relatively few recapturable positions. As such, I see people actually play defense MOST of the time (although even on Berezina you'll see Russian troops running across the bridge at the final objective. For what reason, I have NO idea).
So, I don't think it's a question of not getting the concept. I think it's simply impatience or not giving a damn about one's team and being more interested in killing a bunch of dudes and racking up a high score.