[hahainternet]

The Exomars project will land a 2 metre vertical drill on the surface, sampling much deeper than anything done so far.

It's just a question of priorities. Drilling 4cm into a rock with a specific surface abrasion tool is a long way from plunging a 2 metre metal shaft into unknown terrain. Engineering this so it has no chance of jamming, breaking, overheating etc is not gonna be a trivial task. How do you lubricate a drill without contaminating the environment and with such a wide temperature variation?

[thaumasiotes]

What exactly is the downside of contaminating the environment?

[Ma8ee]

Makes it harder to what compunds are indigenous and what you just brought yourself. No use shouting Eureka over some organic compound in a hole you just spilled oil in.

[careersuicide]

In general great effort is made to ensure that probes don't contain any biological contaminates so if we do discover signs of life we're very certain it wasn't a microbe that hitched a ride.

As far as contamination from drilling (due to lubricants or something else entirely) I imagine that the desire for general cleanliness is for much the same reason. If we're going to spend hundreds of millions of billions of dollars on a probe to another world we want to be really confident anything interesting we find is actually from that world and not Earth.