Less inherent robustness, more just general tendency towards "solving tomorrow's problems with yesterday's technology"
There are companies that take commercial parts and nuke them under a beam at places like TAMU to profile their performance then resell to aerospace. The issue is by the time this flight qualification happens the parts are old as hell.
For small microcontrollers you can just buy modern radiation tolerant chips from e.g. microchip for about $2000 or so per unit, but you need a certain expertise to select the other components you need and design/qualify. Or you can any one of dozens of different cubesat cpu boards, which typically have COTS components and has gone through radiation screening (which is way more work than "nuking under a beam"). These are typically sold for cubesats, but don't meet the requirements for qualification and documentation that is required for an ESA or NASA-related project beyond a cubesat.
There are the famous "old as hell" rad-hard powerPCs which cost $200k, used for historical reasons or extreme environments like a fly-by of Venus. It's true that older chips may be inherently more radiation tolerant due to larger features on the silicon but it's a misconception that CPUs used in space are therefore always old as hell. These are usually selected because they are good enough and to reuse design and qualification from a previous project instead of spending millions and ten person-years on a new design.
But commercial spacecraft these days usually use something like this[1] triple-redundant 50MHz softcpu implemented on an FPGA or this[2] Ultrascale ~1Ghz SOC protected by a secret sauce of separate FPGA for watchdog and overcurrent detection. Both are modern, run linux and use modern development environments/tools. The former is slower but can recover upsets in realtime, while the latter is much faster but needs to reset itself to recover at least some types of upsets.
But there's still a good reason older hardware is preferred. The wider traces on older hardware means less susceptibility to random bit flips due to the higher background radiation. The extremely narrow traces of modern processes are already vulnerable to quantum effects on Earth where radiation is significantly attenuated.
We're talking about softcores here, not fabrication processes, flashing an FPGA with an older softcore design doesn't somehow make it more resistant to interstellar radiation
