I think people were not ready to switch from some heavy established software. Also by that time, costless was seen as less qualitative (certainly because organisations are happy with licences and are always looking for support contract ?) Coming later, when the market(?) is ready, made R more successful.

I think people used to compare to established standard and detractors are always there to point out missing stuffs, giving impression that Octave or Scilab are behind. Coming with a radical different language, R was condemned to quickly succeed or perish, and anyway avoid direct and frontal comparisons.

But R is itself a reimplementation of S.

Same reason people use Excel and not LibreOffice. Until you can promise 100% compatibility, users are going to run into something that breaks vs the original platform. For real business processes, the licensing cost is not worth sweating about the problem for parity.