The problem with double round robin (or long tournament in any non-ko form) is that after some time most players play for nothing or not so much. It's not ideal that the winner of the tournament is a person who scores the most points against opponents who already lost their chances and are not so motivated anymore. Another problem is that it's just another tournament, we have a lot of those in chess. If you want a system to determine the strongest player just look at the rating list which is formed from results of all those tournaments. You don't need to play long additional one.

On the other hand world championship matches have tradition. Most chess fans remember at least some of those. Maybe Fisher's match, maybe Kasparov-Karpov battles, maybe the one when Kasparov destroyed Anand or maybe the one when he banged his head (unsuccesfully) vs Kramnik's Berlin Wall. Matches are popular, they excite the audience, people talk about them long before and long after. The winner guarantees his place in history as a member of very narrow group of players (only 15 of them, for what is over 100 years of world championship history). People yelling "anachronism" don't understand what value this tradition have for chess players and chess fans around the world. Every sport has its own. Tennis has grand slams, football has the world cup, athletics has Olimpics. Chess has world championship matches and please all the armchair revolutionizers leave them alone.