Their previous stadium was declared unsafe in 2005 as it had old wooden stands, the club wanted to redevelop the stands but they struggled to get enough funding for it and even when they did have enough, the mayor/council kept refusing permission to build, saying that it would negatively affect the environment. Also the mayor wanted them to use the council-owned stadium in the city (the one they ended up using for 15 years) instead of redeveloping their old home. They finally started work a few years ago and it’s just been finished.
Interestingly the stadium they played in for 15 years is in La Plata (where they’re from) and was built in 2003, mainly for concerts. It’s actually far bigger than their brand new stadium (it holds 53,000 while their new stadium only holds 30,000). It certainly looks more modern than their new stadium, so I didn’t quite understand why they were so keen to move back. Tradition (they had been in their old stadium since 1907) and a better atmosphere, was my guess. The other stadium they were in was owned by the local council, which I now presume was probably the main reason (I can’t imagine it was cheap to rent and the aforementioned disagreements with the council about their new stadium probably left a lot of bad feeling).
It’s basically like if Ibrox was declared unsafe and instead of letting us fix it Glasgow City Council and the SFA decided not to let us and forced us to use Hampden instead (you can actually see them doing something like that, to be honest). You can understand why they’re so keen to go back.