Because there is none.
Islamism is a sensationalist term used by a 'certain quarter' for their own good. Just like the term Islamist, It too doesn't mean anything. They are inherently vague and offer zero details as if to what does it mean.
- How is being an Islamist different from being a Muslim? - No answer.
- How is Islamism different from following Islam? - No answer
There are genuine terms like Islamofascists that scholars have used to describe people who use Islam and its ideals for a fascist agenda, which is pretty clear. The fact is that if that 'certain quarter' doesn't use these sensationalist terms, they don't get taken seriously. Its like no will pay attention unless you play on people's fears of the unknown.