Strange considering the fact that the ratings for KR1 looked extremely solid unlike today's shows, or was it more about the merchandise not selling or something?
The first
Masked Rider series was not made to sell toys. The toys were made around the show. (When Rider 2 debuted into the show, Popy sold replica Henshin Belts and such.) That was only icing on the cake.
The ratings were lower in the first 13 episodes, because even for a children's program, the show was very drama-like. Also, because it was very different from more popular shows like
Spectreman and
Return of Ultraman, both of which started the 2nd Kaijuu Boom.
Notice that
Spectreman, along with
Mirrorman and
Silver Mask, all had very humble and conservative beginnings, just like
Masked Rider did, and were not much different in evolving from hardcore sci-fi dramas into outlandish kids' fare. But it worked! 1971 was a huge year for the Japanese superhero genre. Not to mention some very tough competition.
The reason the first series had such strong ratings after Episode 13, aside from introducing the Henshin Hero formula into the Japanese superhero genre, was beacuse there were much fewer channels. No different from here in the US. Back then, we had CBS, NBC, and ABC (not counting local networks). Back then, Japan had TBS, NET (now TV Asahi), TV Tokyo, NTV, etc. Today, both the US and Japan have many channels, especially cable networks! As network exec Fred Silverman said in a recent interview, the best a show can get is a 3% or 5% rating.