While debatable, It's often cheaper to pay someone to host than to do it yourself. Imagine a 1 sysadmin small devshop that doesn't want to pay for 24/7 on call support but does have devs working in different time zones. Or a big enterprise that needs support (perhaps someone to blame). Joke about corporate culture if you want, but often it's less stressful to blame a vendor than an employee or the internal culture. It may take 10 minutes to set up. Hours a month to maintain. Weeks to get permission to install it. Time to hire support sysadmin staff. Time to explain why kubernetes/simple vm/heroku/shiny new thing would make hosting it easier.

Why not github? Perhaps the person or org just likes open source. Distrusts Microsoft. Wants the option to self host as a bail out strategy. Or just dislikes github. Competition is great.

This argument applies to most open source apps with hosting options. I'm a fan of this model.