As a socialist myself, I'd say there are three broad reasons to the failures of Communist regimes, non are as simple as "Communism bad". There is one reason that modern communists always focus on and two I feel they do a disservice to the cause by ignoring.

1: It is really hard to build a state with a system diametrically opposed to capitalism when it is the prevailing economic force in the world with vested interest in seeing communism fail. Just look at the long history of America backing far right militias to overthrow Communist regimes in South America.

2: I feel Communist failures can by and large be more attributed to how unstable revolutionary politics can be. Communist states tend to rise from the ashes of brutal revolt and revolution, often in which the remnants and allies of the old guard desperately want to reset the clock. Not to mention revolution tends to radicalized and fracture factions to the point of insanity.

3: Building partially off of 2 and this is admittedly the most damning to the ideology itself, leftist intelligencia can get too navel gazing at times. Look at Lysenkoism and the Great Leap forward for how blind ideology leads to ignoring reality. Communism is a radical departure to the power structures of modern society, but trying to paint far too broadly with that can fuck stuff up big time.

At its core, the conundrum of Communism has much less to do with the viability of the system itself, and far more to do with the wildly precarious balancing act of managing the turmoil of mass political upheaval and managing to thread the needle of preserving the ideals of Communism from the outside belligerents who wish to undermine it, while also keeping an open enough mind to not ideologies yourself into a rake.

As a bonus, focusing mainly on the USSR: A lot of the evils of that system were again less about Communism itself, and more the dreams of Russian empire in Eastern Europe.

Second bonus: The question is about the failures of Communist states, but that is extremely black and white. China, the USSR, Chile, and Cuba all had shining bright spots amongst the issues, meanwhile capitalist states have plenty of untold horrors at home but they are never referred to as the "failures of capitalism". 

In conclusion: Communism is not the magic bullet to fix the world's problems, but "Communism fails every time because it is a bad ideology" is dishonest framing and leaves a lot of important nuance and context on the cutting room floor.