Why did Russia fail in the beginning against Germany?

The main question I'm asking after today, is why did Russia fail in the beginning? Russia was the red army, the big army and they were so strong, but how could they lose to Germany? I know Germany was very strong and undefeated at the time, but Russia had homeland advantage, and they should have prepared better. They have vast land, and they probably could have made alliances with the allies for backup, but instead they took no help and got utterly destroyed by Germany. Why hadn't Germany prepared better? Was it really necessary for Stalin to complete the purge on his army officials? Russia had lost its power when it lost its army leaders, and it could not fight Germany. Why did no one help Russia when they were in trouble, they could have used Russia for even an ally?! Russia was hopeless once Germany took over, half their military gone, and half the country now in Germany's possession. My biggest question is why didn't Russia ask for help?