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.