If you're going to survive on Mars, you already need the ability to sustainably disconnect yourself from the surrounding atmosphere (we don't know of any virus that's as lethal as lack of air), so just apply your putative Mars tech to Earth.
Food shipments? Water supplies? We're talking about the kind of technology that would allow you to live on Mars. If you need to rely on those, then 1) Your Martian colony is not really independent and thus doesn't carry any survival benefits, and thus 2) You're wasting resources shipping them to Mars. If you don't need those for your Martian colony, then why would you need them for your Earth colony?
If Earth were in a situation of having to segment populations to prevent the spread of disease, all it takes is one little mistake to wipe out an entire intermingling group of people. I'm not saying we wouldn't have the tech to prevent that from happening, but humans are fallible creatures. Is it worth risking the entirety of humanity when we could have a remote colony that would be very well protected from those types of events?
Very well protected? The danger specifically from viruses might be somewhat less (though if we're allowing people to travel there, it doesn't seem like much less of a concern), but any other danger you can imagine would be much greater. The remote colony would be no more protected from disaster than one on Earth. On the contrary, since the surrounding environment would be irreconcilably hostile, it would be constantly at risk of one little mistake wiping it out.
