Because you never wrote it. The curse of open source.
By the time github came about, git already had a lot more mindshare than hg. While hg wasn't yet clearly an also ran, it was clearly the less popular solution, and many open source tools started supporting git but either not supporting hg, or if they did it was an afterthought and often the maintainers broke something hg without noticing and then releasing with broken hg support.
yep, that's what i used to use, but it was shut down and i don't really know why - would it have cost so much to have kept it open? dunno - i really know nothing about the economics of keeping such things going.
I don't recall if this was before or after Atlassian bought Bitbucket, but they had a number of issues keeping the site running. There where so many outages, the site was frequently slow to the point of being useless. I know because we where a paying customer. I suspect that they had to few paying customers and because the service was so prone to outages they had acquired a bad reputation so they couldn't attract new customers. Maybe if Bitbucket had been a Silicon Valley company they would have had access to funding, allowing them to grow in the same way Github did.
When it worked it was fine, but in the end you just got way better service and performance on Github.
The current iteration of Bitbucket isn't even Bitbucket, it's Stash. That works really well to, if you like Atlassian tools.