As I understand it, part of the problem is that the vast majority of components simply don't exist in rad-hardened versions. The cost of having a hardened part manufactured and qualified is high, and demand is low; only a few manufacturers bother.
The idea being that failure is bimodal: most stuff fails very quickly, _or_ after a long time, but rarely in the middle. So they just do 100 test cycles in a messy test environment.
ICs for "normal use" also got tests like this, but generally the test suite was a bit nicer. The rad-hardened ones got way nastier tests done to them, and the yield was (of course) much lower. So you charge way more for them.
Radiation hardening is, well, harder. It typically means some combination of different semiconductor processes (often silicon-on-sapphire and/or larger feature sizes) and added internal redundancy (ECC memory, TMR, watchdogs, etc). All of these require that the rad-hard parts be manufactured separately from the commercial ones; some require that the part be designed separately as well.
I just might not be sure on the "space-rated" things being "radiation-hardened" or if it's more just "well they're gonna be more tested".
Sometimes the vendor will add a little more to the substrate that doesn't increase cost much but does provide a little bit more radiation tolerance. Other times there are certain things the customer must do like constantly rewriting registers. It's not perfect but you can save a lot of money taking an off the shelf TI part and flying it versus spending tens of thousands for something of similar performance but with a guaranteed level of radiation hardness beyond what you actually need. I've used a commercial TI ADC that works really well in a radiation environment (perhaps as long as 5 years in GEO) despite not being sold as such. We just decided to screen it to see how well it performed and based on that test data, we made the decision to fly it. I'm not sure who's idea it was but something about the design of the chip led us to believe it might perform decently. TI was not involved and was at first a little concerned when we told our sales rep we were planning to fly it.
Older chip designs use larger features that generally perform better in a radiation environment than newer designs using smaller features. An old 74-series logic chip might actually perform just fine when it comes to heavy ions but may not live as long as something like a 54-series that has a larger substrate that can absorb more gamma over time.
Another method is flying unscreened radiation hardened parts, they were built to a certain standard but you buy them without all the extra testing to save a few grand because you trust the vendor has a good process.