Why was the Soviet Economy so bad during the Cold War?

The question could really use some clarification and/or specification with regards either to the scope or the time-frame of the inquiry - the assumption that the Soviet economy was completely "bad" throughout the Cold War is unfounded. There's plenty of data&graphs here, particularly of note this one, detailing GDP growth between various countries.

The graph shows the GDP growth to be fairly low, but it is still growth and is more akin to the rate of a fairly mediocre economy in these specific conditions and circumstances.*

When looking at why Soviet Union would be perceived as a bad economy, we would be lead to GDP in real terms when compared to the US (the main comparable super-power of the time). The issues here are obviously more complex than can be put in 5000 or so characters, so this is one of the best books on the topic.

GDP in real terms

... is first and foremost affected by the starting figure of the timeframe measured - in the case of the USSR, their GDP was significantly lower than that of the US in 1945. That can be attributed to the fact that most of what become to be the USSR, incl. the main economic and industrial centres (Moscow, St. Petersburg, Volgograd) were significantly damaged in WWII, whereas most of US was reasonably untouched by the actual war. Add to this significant losses in civilian population, and it becomes quite clear, why WWII was a net negative for the USSR GDP, and a fairly significant net positive for the US GDP. There are obviously various pre-war factors as well, but discussing them at length would take a bit too long.
... is also obviously affected by nationalisation of most enterprises. Nationalisation of enterprises mostly meant that the incentitive to minimise costs and maximise profit at a managerial level was essentially non-existent, as any profit made or costs ecaded would go back to the state and not the owners and managers of the business - it was more beneficial to use any surplus to produce extra goods covertly and sell them on the black market for personal gain (this happened in most producing industries on most levels, across the USSR).
...and the rigid planned economy that was implemented in the USSR. The rigid planned economy (incl. the five-year plans) just added to the previous, by limiting what companies in different industries could and had to produce - this often lead to situations where companies had to produce goods that had become obsolete through technological innovations, because the plan mandated so (not fulfilling the plan was not an option in most cases - coming even a bit short, even in the case of extenuating circumstances, could lead to changes in management or closing of factories and businesses) or where businesses had to produce goods that could have more effectively been produced in other parts of the Union (e.g. Nikitak Khrushchev tried to improve the Soviet economy by mandating the cultivation of corn across the USSR, even in the Baltic states, where there is much too little sun and too much rain for it).
...last but not least the lack of international trade, the USSR took part in - for most of it's history the Soviet Union actively disincentivised and limited any international trade with other countries, esp. Western democracies. This is not such a major issue with regards to import, as the then USSR had plenty of raw resources to sustain it's population and industry, but the influence lack of exports had is thought to be much more significant - most goods with significant added value, that could boost the economy if sold outside, never made it out - most of the small amount of exports was either unprocessed foodstuff or other raw resources.

^*The ^fact ^that ^the ^indexed ^growth ^rate ^is ^similar ^to ^that ^of ^the ^US ^and ^Switzerland ^is ^a ^big-big ^economic ^can ^of ^worms ^and ^discussing ^and ^explaining ^that ^is ^a ^bit ^too ^long ^a ^discussion.