I’m sure there is someone with more technical knowledge on this, but in a nutshell content blockers add a bit of overhead to Safari as it is basically just string matching that needs to be executed fast, for every page load. This is something the 64-bit processors are much better at. Content blockers were designed for speed and efficiency so it makes sense that they would restrict it to the hardware that actually delivers that.
Also note that Apple does enforce a size limit for the block list, so I suppose there needs to be a baseline where the performance remains adequate if that limit is hit. So even though ‘light’ adblockers could run just fine on 32-bit devices, maybe the performance is bad on large websites with an extensive block list. This does have an impact on the user experience and may be of particular concern to Apple.