It's not that there aren't alternatives. There are plenty. (And how'd you miss gerrit!)
It's that github has become a platform. More users = more success and github is commercially motivated, so it is motivated to create a proprietary platform which is incompatible with other proprietary platforms. There are many built-in features that are hard to fund if you're not successful (have enough revenue), and there are many 3p integrations that 3p won't bother to implement for non-github targets. Thus cementing a github monopoly.
It's that github has become a platform. More users = more success and github is commercially motivated, so it is motivated to create a proprietary platform which is incompatible with other proprietary platforms. There are many built-in features that are hard to fund if you're not successful (have enough revenue), and there are many 3p integrations that 3p won't bother to implement for non-github targets. Thus cementing a github monopoly.