Is there a simple explanation as to why the plant was so expensive?
Because it is big and complex?
There is a lot to the facility. There are multi-stages to the process. The construction site used 40,000 people and was the size of Central Park. It is one of the world's largest "single" facility.
But given the price of Natural Gas in Qatar, it makes economic sense to spend this much and essentially export very high grade crude (gasoil) versus spending less but still significant capital to export lower value LNG.
Pearl will process about 3 billion barrels-of-oil-equivalent over its lifetime. The return on investment will work. I don't think we will see the lasting return of $20 oil that would make it uneconomic.