Rocky Arroyo was a popular place for lots of reasons. It's pretty. It's wild. Being about 20 miles away from Carlsbad, it's certainly "out there" and not a place that ever sees much of a LE presence. At least not back in the 1960s.
The only thing wrong with Rock Arroyo is it's pretty, wild and a fun place to be. This makes it a popular place for school/church parties etc. I went there in such groups several times growing up. Naturally during such events there are people everywhere and not the best of places for double murders.
But just what constitutes "rocky arroyo" is up for considerable discussion and might be thought of as a rather lengthy area of that little creek. Some of it is more remote than other parts and much less used.
A school night would probably be the best time for privacy. And I can see RA as being the intended destination of the killer. But for some reason he cut this idea short and just got rid of the girls ASAP and barely off highway 285.
One church trip I went on to the area when I was in school we played "Hide N Go Seek" after dark and with all the big rocks, undergrowth, etc it was a devil of a place to find anyone after dark. Ironically, if Patty and Mattie had been taken to RA and had bolted from the car and run like crazy in opposite directions, one of them might have gotten away...assuming just one killer was involved.
Since DB had an accomplice along on his first rape, I have some question whether he was along with both girls and the "other guy" lost his nerve at some point But at this time all this is just conjecture.