Considering the needs it’s catering for, the available options make a lot of sense.
However, think about this in terms of the people who are in charge of deploying the OS. They need to test first, and considering that they have an interest in a Linux distro, they are probably used to dual booting already or they have a lot of experience with installers that are present in most Linux distros - which allow installation into a different position - and so they need to be able to see it’s usefulness and test it.
Windows users aren’t often interested in a Linux distro, it’s people who have used Linux that are interested. Most of these are dual booting already with another Linux distro such as Ubuntu, and the inability to test Endless really isn’t helping for the exposure of the public to Endless OS (or Endless OS to the public).
Please work on this. Flexible installation ability is one major reason people try Ubuntu (and Ubuntu-based distros), because they can do it without affecting the current state of their systems. If you want Endless to be popular, let people try it!