Tantra popularity

As you have elaborated all kings of those times adopted and followed a tantric system. Can you throw some more light on the motivations behind it and how exactly it helped strengthen the state?

It is difficult to answer this question briefly. I don’t know for sure why there was a growing interest in royal tAntrika rites. Based on the options if offered my mundane level proposal is that it could be adopted by a much broader swath of the warrior “guild” at many levels (sAmanta etc) rather than just the monarch (an emphasis of the vedic system). Its practitioners, e.g., saiddhAntika shaiva-s were very well organized for institution building and this was a major service for the state.