Because failed ones don't last long enough to be remembered. There was a guerrilla insurgency in Iraq after Saddam's government fell, for example, which folded pretty quick.
But the point of this is that unpopular occupation forces trying to impose their will never really works in anything even remotely like the long-run. The only times it can work is when a genuinely popular (and not artificially propped up) leader is able to find common ground with the "enemy". For instance one of the best decisions we made after WW2 was with the leader of Japan, Emperor Hirohito. We actively avoided not only attacking him, but did not try him for war crimes, and let him retain control of the country after the war. He chose to lead Japan in a different direction following their defeat, and everybody came out all the better for it. It's very likely that had we executed him, or installed a puppet, that Japan's relation with the west would be radically different today.
The Emperor of Japan was part of a very old, very well established monarchy; his legitimacy in the office of Monarch far outweighed that of Saddam's in the office of Tyrant.
In general, the more likely reason is that most people tend to be resolutely incurious about foreign politics. There are, IIRC, 150-ish ongoing armed conflicts in the world right now (and this count is far lower than it was 50 years ago!). I suspect that even reasonably well-read Westerners would struggle to identify even a dozen of them. So people selectively remember the ones that involved the major powers (e.g., Afghanistan, Iraq, Vietnam) while completely ignoring ones that don't (so, how about that rebellion in Aceh that failed...).
Palestine?
Modern Western countries haven't had much of a history of successful insurgencies. Hungary and Czechoslovakia were both invaded by the USSR (along with Warsaw Pact allies), and it didn't result in an insurgency. We shouldn't exclude WWII either, since that was the last time many of Western nations were invaded by outside powers. The resistance movements never seriously threatened Axis rule in Western Europe.
Everyone seems to be expecting a large Afghan style insurgency, but I think it's far from certain that it will happen.
I can think of one, and unfortunately it was the Russians who won it, but Ukraine is much bigger and better equipped.
=> Cunningham's Law (a submission earlier this week) playing out nicely in the replies.
It was a defeat, no matter what coat of paint you put on it.
I don't know if there's the same cultural will in Ukraine to absorb that kind of pummeling.
https://en.wikipedia.org/wiki/War_in_Afghanistan_(2001%E2%80...
Result: Taliban Victory
When your reliable supply lines are all flights, costs will seriously mount up.
Plus, they had stingers. They could raid and retreat essentially forever.
The thing is, if the Ukrainian is really again a Russian regime, and it sounds like they do (if because I'm not them can't speak for them), then Russia has already lost. In time, whatever puppet state that it creates, will fall. America learnt this lesson too many times, most recently in Afghanistan.
The name John Mearsheimer has come up recently and I think one of the key points in his theory is solid: nationalism triumphs everything else, be it America branded liberalism, or communism.
When he says "nationalism", it's actually not what we often think, in his theory it's used in the narrow sense that is the need to determine one's own country's destiny, as opposed to having to obey a foreign power.
Edit: some folks have mentioned several, so let me be more specific: guerrilla wars to expel an invading power. This would exclude several attempted Marxist revolutions, but good points made about Chechnya, as well as Northern Ireland and Kenya which failed to expel colonial powers.