Triple jct cells are insanely expensive. This article is a bit out of date, but the cost difference holds true. When I was working on the solar car for my university back in 2009, the GaAs cells were hundreds of thousands of dollars in total.

https://www.solarreviews.com/blog/nrel-developing-tech-lower-cost-multi-junction-solar-cells

>To give a cost comparison, utility-scale solar projects now cost about $1 a watt for solar power. A gallium arsenide multi-junction solar cell costs up to $300 a watt at this point.