The nature of any political organization is that the commitment only lasts as long as the administration that makes it. And for a big-ticket item like a manned Mars mission, that's not long enough.
Whereas a private venture would have to rely on either a reasonable return on investment within a reasonable time; or the undivided attention of uber-rich hobbyists with money to burn.
There are a couple of things to keep in mind.
First, we're really just talking about the word
manned. Unmanned Mars missions are quite common these days. So any discussion has to focus on the rationale for sending people, rather than robots.
Second, if we decide to do a manned mission, we have to consider the best "business case" for getting it done.
And to be honest, a large-scale government program is perhaps the better business plan, if only because big money is easier to come by, and ROI is not an issue.
The downside of a government program is that it typically has to address some long-term threat, rather than some nebulous feel-good pabulum about mankind's need to explore. Which is why, for example, the early Space Program was undertaken to address the threat of Russian dominance in space.
The downside of a private program is that the ROI just doesn't seem to be there, and hobbyists generally don't have billions to toss down the rathole.
