Basically, there are some blocks on some networks, it's normal. For example, your boss doesn't want you to see certain things while you're at work. So, he blocks some sites. Alternative links are for this purpose, technically I can't explain what changes. However, something changes in each link that some allow you to access.
For BC.GAME's alternative links, our friend @Cantsay helped with the alternative link:
I know about the official website that lists all the mirror links, but I'm not sure how access is restricted by the ISP. In my country, India, some members have reported that BC.game is inaccessible. However, as an Indian living in India, I was still able to access the website using the same link. I asked them which ISP they were using, and to my surprise, they were using the same ISP as me in a different state. I find it hard to believe that it's solely the ISP causing the problem; instead, I think it might be the users' internal firewall configurations on their mobile and laptop that are causing the issue. I may be wrong, but I have a feeling that I'm not, as many gamblers try to conceal their gambling habits using in-house firewalls, also known as antiviruses.