The article doesn't really lay out any particularly difficult, let alone insurmountable, problems. In fact we even had plans to make it Mars before 1980 in the 60s drawn up by the exact same man who got us to the Moon. The reason we're not a massively multiplanetary species already is simply because of politics. Nixon defacto cancelled the human space program in 1972. After that we had to wait for the economy to develop to the point that private entrepreneurs could viably send us back to the stars, and that's where we are today.

Had we maintained the political will though there's no real reason we wouldn't have had a colony on Mars decades ago.