The California redwood (Sequoia sempervirens) natural habitat is essentially coastal northern California. According to Wikipedia, the northernmost point they can be found is only 15 miles north of the California-Oregon border.
Is there a reason it doesn’t grow further north along the coast?
Too cold and wet, plus wrong soil conditions. Redwoods are quite picky about their ecological requirements - mild tempertures, wet Mediterranean winters and abundant summer fog ( more important in the southern end of the range ) are necessary. In addition they tend to be confined to serpentine soils and a glance at an Oregon soil map will show that coastal serpentine stops not to far north of the border:
In general southern Oregon is grouped into the California Floristic Province, because of similarities of ecology:
http://www.nmnh.si.edu/botany/projects/cpd/na/map4.htm
It gets quite a bit colder between Brookings, OR and Crescent City, CA.
Plus, the Coast Mountain Range begins right around there, and Douglas Firs start taking over.