I'll try to keep this as organized as I can. I've been jumping back and forth as I write this, so I apologize if it's disjointed. I’ll compare European partisans with the Soviet experiences in Afghanistan and the NATO experiences in Afghanistan.

For starters, partisan is a French word and the French fighters called themselves partisans. Since France was a western power, it's understandable other western powers would refer to them as partisans and then extend the use as a blanket term on all of the European resistance movements.

Guerrilla wars and insurgencies are different by definition, but sometimes the titles can overlap.

Guerrilla wars describe HOW the war is fought while insurgency is WHO is fighting, but since tactics and strategies often overlap, you'll see the titles used interchangeably.

Many partisans in Poland and France probably didn't think they could overthrow the German government on their own. They used guerrilla tactics to indirectly slow down the German warfighting capability by destroying supply depots and railroads. This indirectly hindered the German ability to fight at the front because they had to dedicate troops to the interior to deal with partisans. Partisans were willing to keep killing Germans or wait out the German forces until the Allies showed up. In the 1980’s, Islamic groups sought to overthrow the Communist Afghan gov’t and the USSR intervened on the gov’ts behalf while the US went in in 2001 to go after Al-Qaeda and the government that was harboring them, the Taliban. In the 1980’s, the varying groups fought against the Soviets and some controlled entire swathes of land where they could effectively rule the towns. When confronting the Soviets, they often did quick hit-and-run ambushes, IEDs, or mortar attacks after realizing they could not go against Soviet armor and gunships. After the Soviets left, the Islamic groups were able to gather in larger forces and fight more conventionally. Since they were still unrecognized groups seeking to overthrow the Communist Afghan government, they were insurgents who had switched to more conventional tactics. Against NATO, AQ/Taliban fighters hid among the locals more since NATO forces spread out into more villages and towns in an attempt to stop the Afghans from gathering in large numbers. AQ/Taliban rely on ambushes and IEDs to inflict casualties. Against both the Soviets and Coalition, the mujahideen sought to expel the invaders and dismantle the Afghan governments to install their own gov’t. Even today, the mujahideen are waiting for the western forces to leave.

Referring to a group as insurgents also delegitimizes them since insurgencies, by definition, are unrecognized groups seeking to overthrow established governments.

I would say viewpoint and the media plays the biggest role though. By definition, many of the titles overlap. The west viewed European partisans as fighting back against the evil occupying force that was the Third Reich and who needed support, so partisan was viewed positively. The Germans certainly referred to these partisans as insurgents. During the Cold War, Che Guevara and his “guerrillas” were fighting for Communism, so guerrilla had a bad connotation associated with it. In the 1980's, the American government referred to the Afghan mujahideen fighting the Soviets as "freedom fighters" fighting against Soviet invaders and the Afghan puppet regime. Nowadays, since the Afghan gov’t is an ally (if you’re a westerner), the Taliban/AQ are viewed as insurgents since they want to overthrow the Afghan government. Even though many of them are farmers fighting against what they see is a foreign invader.