The technical reason is that trying to combine every possible proof of work based quorum into a single chain does not scale. This is not a matter of personal preference, it's about solid engineering.
Agree with [mike]. It is pretty obvious that allowing domain name registration info in the block chain does not scale. If you open the doorway to domains, other potential applications using P2P could also do the same causing the chain to bloat. It truly is about solid engineering.
If the names are free then somebody will just do a dictionary attack on the network and claim all useful names.
This is pretty obvious. Registering a domain must cost something so all the names will not be taken. One cannot just get it for 'free'.