It generally boils down to setup time.
This business analogy would be that you are looking at something close to a fixed time requirement that isn't going to change much regardless of the size of your project or traffic level. If you've got a project where you are actively working, paying this cost is very low-investment and high reward. If you have a side project or are splitting time on multiple projects then it's time you are spending not focused on the project itself.
If you've never done it before then the up front investment and uncertainty can also be a deterrent.
When you are looking at a language like Elixir where you are already coming into new language, environment, tooling, expansive clustering options and deployment options, OTP and the like...having one less thing to learn is a huge benefit.
It's worth learning eventually, but it's very understandable for people to want to have options that don't require it up front.
In my case, I did PHP professionally for about 4 years before I signed up for a Slicehost account and started tinkering with my own server setup for personal stuff. About a year later I started running my own servers for everything and still rabbit holed down options for log management, monitoring, debugging, keeping things up to date, security patches, rotating SSL, managing user permissions, etc. Most of the reason that I got into that was because back in 06-07 you had to dive into server configure heavily to performance tune PHP.
It was a great experience for me to learn...but it was a lot of time and I'm still learning. Its not for everybody and services like Gigalixir and Heroku are invaluable in helping people bridge those gaps.