Probably because scrapping it would've cost a small fortune and what's left over just wouldn't be worth it.
I'm surprised that they didn't post a guard there with clean up duty to at least keep it all from rusting in to the ground.
Wonder how much it would be to buy and whether you could get enough visitors to the "secret Russian space base" to make it pay.
Scrap collectors on the street don't charge for their time. It would cost the Russian government more in wages and such to scrap the site than they would ever get back out of it.