Creating new habitable space will be expensive it will be like living in a submarine or an Antarctic base during winter. Forget about having large space for entertainment like a football stadium or a theater. I think that after a few months of that regimen people will start to appreciate more life on good old Earth.

It may, in fact, require more resources for one person to live on mars than one person can produce with our current (or near future) level of technology.

I think just building the energy infrastructure to do energy expensive things like make steel or silicon ingots will be very difficult. Not to mention that there is no source of petroleum on mars, so plastics are going to have to synthesized using other materials. The difficulties are legion once you get down to the details.

The weak gravity is the real killer, colonists bones would decay, the calcium would leach into the bloodstream causing serious health issues. Muscles would atrophy. The colonists' sense of balance and motion, evolved to Earth gravity, would be constantly disorienting. Low gravity has a lot of negative health effects.

Mars also has nothing like the Van Allen belt, it's bombarded with radiation that colonists would constantly need to be shielded from.

Mar's atmosphere is poisonous to humans, and has a much lower atmospheric pressure, so colonists would need to have a constant protective bubble protecting them from the world they were on.

Mars hasn't got liquid water. There is water there, but it would require a lot of energy to get it to the point of being usable.

Energy is another issue. Solar energy could provide some energy, though it'd be massively expensive to get enough arrays to support a colony with the required infrastructure. In theory they could create some kind of nuclear energy plant, though the logistics of transporting that to Mars and maintaining it are staggering.

The psychological impact of taking an animal evolved to live in Earth conditions and put them in a completely inhospitable environment deeply dependent on a very complex and expensive infrastructure, with disorienting gravity, health decay, very limited variety in food, and other likely spartan living conditions a Mars colony would be completely hellish for most.

There may be ways to manage these problems, but the millions of dollars spent per colonist to get them there along with the millions spent per colonist to sustain them are a pretty serious impediment. A colony would need to ultimately be economically independent to make it worthwhile. Mars has some resources that might be valuable to Earth, but whether they'd be viable to support a colony needing such an expensive infrastructure to survive isn't a guarantee. Would humanity be willing to pour trillions upon trillions into a Mars experiment in the hope that the experiment might work?

The technical issue is not the matter. It's the sheer amount of resources for which the overwhelming majority of the human race will never see a benefit from in their lifetimes.