Why did Biosphere 2 fail?

Biosphere 2 was an ambitious project that involved an airtight, three acre, sealed facility with various ecosystems like a rainforest, savannah, and ocean. There were two separate missions involving a total of eight people sealed inside for as long as two years. What followed was a nightmare situation of low oxygen, gasping for breathe, starvation, and declining health. So why did Biosphere 2 fail?
There was no sufficient room to grow enough crops for adequate calories. The food crops were too slow and labor intensive to be worthwhile. The coffee bushes alone took almost two weeks to produce enough coffee for one cup. The people inside ended up as subsistence farmers surviving on beetroot and sweet potato.
Perhaps even worse, the oxygen levels decreased faster than anticipated as carbon dioxide built up. The air we breathe is normally 21% oxygen, but the biosphere at oxygen levels as low as 14.2%. What result is difficulty sleeping. It was like mountain climbing. It became difficult to finish long sentences. It was discovered that there was an issue with the concrete structure of the biosphere which made it difficult for gases to properly circulate.
Works Cited:
https://dartmouthalumnimagazine.com/articles/biosphere-2-what-really-happened
https://www.theguardian.com/film/2020/jul/13/spaceship-earth-arizona-biosphere-2-lockdown
https://www.scientificamerican.com/article/biosphere-2-the-once-infamous-live-in-terrarium-is-transforming-climate-research/