Reentry is easy; You just plop right into the atmosphere & ocean, barely any work needed. Getting to the ISS' orbit is hard; Coming in from interplanetary space, you're looking at >4000m/s adjustment that needs to be made to reach orbit. Making it using fuel requires several times your probe's weight in propellant, and making it using aerocapture / aerobraking requires basically the same heat shield as when you do reentry (slightly thicker), but with high-precision orbital windows that need to be hit very precisely to make the rendezvous, involving likely hundreds of m/s dV capability since those windows are transient solutions of a dynamic thermosphere.

Edit: While technical specs are always hard to find, one is left to believe that rather dramatic increases in capability per mass of heat shield material have occurred since the Shuttle program and then under SpaceX.