Because in most cases there was no choice between becoming a partisan versus remaining in the regular military - the partisan movement was formed by men and women living in German-occupied territory, in areas that were often far beyond the reach of the Red Army. The nature of partisan warfare is that it is generally waged by irregular resistance movements living behind enemy lines. The Soviet partisan movement of World War II is a quintessential example of this type of struggle.

The partisan movement came into existence because of the Red Army's initial defeat during the German invasion of the Soviet Union in 1941. Indeed it was the total failure of the regular Soviet military to stop the Germans that led to the existence of a partisan movement at all. If the Soviets had halted the Germans at the front line, then there would never have been a partisan movement. But Operation Barbarossa shattered the Red Army, and soon vast areas of Soviet territory lay behind German lines. Moreover, German tactics focused on piercing enemy lines with fast armored columns, leaving the slower-moving infantry to mop up Soviet troops who had been encircled. This meant that many thousands of Soviet soldiers found themselves suddenly stranded and cut off from the rest of the Red Army. A Russian soldier might have started his day in 1941 on the front line, and by evening found cut off and surrounded, miles and miles away from his comrades, without ever even moving an inch himself. I'll quote Al Stewart's song 1973 song "Roads to Moscow," written from the perspective of a Soviet infantryman during the German invasion:

They crossed over the border, the hour before dawn

Moving in lines through the day

Most of our planes were destroyed on the ground where they lay

Waiting for orders we held in the wood

Word from the front never came

By evening the sound of the gunfire was miles away

The partisan movement became steadily more organized over the course of the war until Soviet territory was fully liberated in 1944. Soviet leadership encouraged resistance during the dark days of 1941, but the actual organization of partisan groups depended on local initiative. People caught behind German lines had a long legacy of guerrilla warfare to draw on. Soviet history textbooks celebrated guerillas who had fought Napoleon during his 1812 invasion (the original Patriotic War) and Bolshevik partisans who had fought the Whites during the Civil War. Early resistance in 1941 coalesced around Red Army soldiers left behind German lines, as well as local Communist Party members. But it also emerged spontaneously from local populations. The Axis occupation of the Soviet Union was extremely brutal. According to Nazi racial logic, Slavs were an inferior people destined to be colonized and exploited. German authorities were correspondingly callous and indeed inhuman towards the population in occupied territories. Many partisans explained that they joined resistance groups because they felt it was the only alternative to starvation and murder by the Germans. Others joined to avenge their loved ones, or from genuine ideological and patriotic motivation. In some cases the Soviet government organized party members in advance to "stay behind" as partisans when the Germans arrived, but in general during the early days partisan movements sprung up on their own.

Terrain also played a large role, as swampy or well-forested areas made it easy to hide and were conducive to the formation of irregular resistance groups. Soviet historians claimed that there were over 90,000 partisans in the German-occupied zone by 1942, while Western historians have generally argued for smaller numbers such as 10,000, but it's not known for sure. Obviously underground movements did not keep extensive records of membership for enemies to find. Whatever the case there was a large resistance movement to build upon behind German lines by the second year of the Great Patriotic War.

In 1942 the partisan movement became increasingly centralized and organized by the Soviet government. Soviet authorities produced propaganda to encourage people to enlist in underground movements and intimate collaborators, began to provide logistical support to partisan groups, and increasingly provided leadership for partisan groups by training staunch Communist Party members to go behind enemy lines and drum up resistance. This was similar to how the British and Free French dropped agents into France to help organize the resistance there, but in the Soviet Union it happened on a much larger scale. When the Red Army moved forward it also provided support for partisans, dropping paratroopers into partisan zones, and indeed sometimes even infiltrating entire Red Army units behind enemy lines to help the partisans.

On the other hand, the partisans increasingly acted in coordination with Soviet authorities. In the summer of 1942, for example, partisans in Orel and Belarus moved 35,000 men across enemy lines to join the Red Army's ranks. In areas near the front line, or when Red Army troops advanced into German-held territory during offensives, partisans would try to link up and fight alongside regular troops. By 1943 and 44, as the Red Army cleared the Axis out of its pre-war territory, partisan units increasingly merged with the regular army. The same sort of process happened in other fronts like France, where liberation meant that partisans could come out of hiding and begin waging conventional warfare. The partisan movement reached its apogee during the Red Army's 1944 summer offensive code-named Operation Bagration. Partisans provided reconnaissance for the Red Army's advancing columns and wreaked havoc on the retreating Germans, destroying communications, bridges, roads, and mopping up thousands of disorganized German troops making their panicked retreat. It was the culmination of what had been a massive people's war behind enemy lines that had severely tied up and damaged the German war effort for years.

After the liberation of Soviet territory in 1944, the partisans continued to play a role in the war by helping to liberate the rest of Eastern Europe. Experienced partisan detachments were sent to help organize similar efforts in places like Poland, Slovakia, Yugoslavia, etc. to lay the groundwork for the advancing Red Army, and to help Communist resistance movements achieve superiority over non-communist groups.

Tl;dr - Your question is based on the faulty assumption that most people had a choice between joining the partisans and the the regular Red Army. People joined partisan movements because they were caught behind enemy lines, where Soviet government and rule of law had been replaced by extremely brutal and arbitrary Nazi occupation. They joined to defend their homes, resist the Axis regime of terror, and continue playing a part in the armed liberation struggle where the Red Army no longer could help.

Sources:

Alexievich, Svetlana. The Unwomanly Face of War: An Oral History of Women in World War II. New York: Random House, 2018.

Grenkevich, Leonid D., and David Glantz. The Soviet Partisan Movement, 1941-1944 : A Critical Historiographical Analysis. London: Routledge, 1999.

Howell, Edgar M. The Soviet Partisan Movement 1941-1944. Washington, D.C.: Department of the Army, 1956.