Economic systems absolutely affect development, but again, you’re comparing a country that was a backwater nation completely undeveloped come the start of the 20th century with a country that has always been at or near the top of the list of industrialized nations. The starting points aren’t even in the slightest.
Secondly, the banning of alternative political parties was indeed antidemocratic, but the party didn’t select who you could vote for. Factions were banned by Lenin, supposed to be temporary, but this continued until 1989.
Historical accounts actually disagree with you saying candidates were preselected. en.m.wikipedia.org/wiki/Soviet_democracy Pat Sloan in particular mentions that anyone could be elected, at the local level. Perhaps what you’re referring to is that those above the local Soviets were made up of those elected at local Soviets, and thus people couldn’t directly run for higher Soviets? Either way, definitely flawed, but not the same as what you’re saying.
Democracy is a sliding scale, I would say the Soviet Democratic model was still democratic, but less than many other countries. The US is technically more democratic, but many absolute positions cannot be voted on, such as the Supreme Court. There isn’t a currently existing country with fantastic democracy, unfortunately.