This article from Medium provides in-depth coverage of the various reasons why. It goes into all the scientific details of the problems any manned Mars mission would have to face, the psychology of signing up for interplanetary journey to essentially spend the rest of your life on death row, the flimsy and slapdash nature of, and false claims associated with, the Mars One company itself. But what makes it a particularly good article is that it grounds its narrative in the story of one of the applicants for Mars One, an Australian guy named Josh: Worth reading the whole thing.
Don't know about you, but I don't live in a tiny bunker with no prospect of ever seeing anything else again. "Death row" is imprisonment, not just mortality.
Mars One was never going to raise the money to get there in the first place, regardless of whether any country could tell them they could go or not.
Are we really suprised? Any manned mission to Mars let alone pernament would cost tens of billions if not hundreds of billions.
Some good links if you'd like to do some more reading on space exploration, and why settling an offworld colony, even to a place as relatively close as Mars, is extremely difficult and expensive, if not downright impossible with our current capabilities: Why is it so hard to travel to Mars? How many people would it take to build a self-sufficient offworld colony of our current technological level? (In case you're wondering the quick answer: ~100 million.) Why is space colonization so fascinating to a certain subset of people? This piece on the space shuttle is also pure gold. It encapsulates quite well why manned space travel is so difficult, not merely because of technology, but also politics and bureaucracy.
The inevitable failure aside, I don't think that the original plan was a suicide, life on death row or even a one-way trip. Not because it was never going to fly, but there is still a connection. The entire thing was so extremely optimistic, that if it were to ever succeed, options have immediately surfaced to take care of the astronauts, to turn them into genuine colonists, to send them company and ultimately to plan them a return trip home. At worst, if they are on Mars with extremely bad prospects for the future, that would alone invite enough interest and sympathy to find the money to take them back. But that's the worst option. Right now, SpaceX have a much more realistic plan – it involves development of spacecraft that far surpasses the capabilities available now (any plans with what we have are far-fetched, at best). And I don't think that would be enough without breakthroughs in extraterrestrial mining robotics, and a solid plan for local resource utilisation on Mars. Or something. But if the immensely more crazy Mars One succeeded, so would SpaceX a few years later. The latter at least knows what they are doing – as much as it is possible when it comes to space. And if SpaceX flies too, there's nothing to stop them from landing at the same spot or even offering return flight seats for the people already there. So the banishment/death penalty side of it was a bit overblown. And, I mean, it's not a suicide mission if it is next to impossible to ever embark on it.