There were some compatibility problems that required genetic engineering of the pig. I don’t remember specifics, but there was talk about potential dormant viruses in pig DNA that need to be removed first, possible problems with glycolsylation (sugar chains on the proteins outside of cells) and maybe more. The article also mentions that the pig had some human genes, I’m sure those help compatibility too. So many changes would be next to impossible to do until relatively recently, before use of CRISPR-Cas9. Also, it must’ve taken some time to certify the procedure. That’s why it took so much time since the topic was hot.
I didn’t read the original paper yet, perhaps it’s there, but it isn’t in the linked article nor its source Ars Technica article. Can authors themselves upload their papers to these archives, and if so, how to do it correctly to make it findable both by DOI and other means? Does anyone know?