Why then not develop this in the first place instead of first building a military vehicle and maybe later reusing it for different purposes?

The military need it is being built for is "casualty evacuation and cargo resupply". The only thing really distinctly military with regard to these is that the military happens to be a large single customer who, if they purchase it, will secure the future of the company in one fell swoop.

Which is one reason to focus on military first.

Its not really "reuse for different purpose" when you also sell the same thing to civilian customers who have the same kind of needs.

The simplest reason is that the military has the funding and will take risks on new things that the others will not. They also stress the heck out of things, and the military is willing to develop and iterate on things that civilians would get sued over.

Funding