Hi Mate - good question.
I would say skills, time, and data/statistics.
If you can’t code your strategy development process is gonna be manual. And that is just such a long painful process. I’ve spent endless amounts of time in spreadsheets collecting data around strategies I’m working on, as I am sure many people on here have too.
And then that manual process makes it difficult to collect good valuable stats over a long enough period in a quick enough manner. It’s just impossible to be efficient in those circumstances.
But I think strategy development is a skill on its own. Too many people put themselves in a box of “I’m an XYZ trader and I trade XYZ strategy”.
Be curious find a scenario you like the look of, ask questions about the market, and then find the answers in the data.