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For the same reason its hard to charge a Mafia kingpin. Everything is strongly implied, never written down, and vague.
Unless someone turns tail and testifies.
The panel also recommended that John Eastman, a lawyer who was essential to Trump’s plan to stop the election certification, be prosecuted for obstruction and conspiracy to defraud the country.

The DOJ has no legal obligation to act on the referrals, but the Jan. 6 committee already has a sense of where things are going.
Eastman had two search warrants executed by the FBI months ago and is being targeted by the DOJ and Fulton grand juries. He's also been the center of a lawsuit where a federal judge has said that Eastman and Trump "likely committed" multiple felonies.
Many correspondences revealed/leaked from Eastman's team show that they knew their actions were illegal, and there is a very important piece of evidence regarding Trump's intent:

“Although the President signed a verification for [the state court filing] back on Dec. 1, he has since been made aware that some of the allegations (and evidence proffered by the experts) has been inaccurate. For him to sign a new verification with that knowledge (and incorporation by reference) would not be accurate,” Eastman said.
“President Trump and his attorneys ultimately filed the complaint with the same inaccurate numbers without rectifying, clarifying, or otherwise changing them,” Carter wrote. “President Trump, moreover, signed a verification swearing under oath that the incorporated, inaccurate numbers ‘are true and correct’ or ‘believed to be true and correct to the best of his knowledge and belief.’”

If the DOJ brings the Jan. 6 investigation to trial, I think obstruction and conspiracy will take center stage. Incitement/insurrection may come into play, but will have a much higher bar for intent.
Eastman, Clark, Navarro, Giuliani (all of which have had search warrants by the FBI), along with the fake elector scheme and state pressure campaigns - will be huge pieces in a potential prosecution. >

What do you think the bar might be for intent? True that there is very little case law, but I can’t say any reasonable person would say that a President doing absolutely nothing over the course of three hours to stop an insurrection while it is taking place, and speaking of pardons after the fact don’t fit the definition of providing aid and comfort. >

The insurrection charge would be tough to prosecute. I think they would almost need written directives to spur violence or something similar to make it airtight.
When they try to piece together statements like "fight like hell" and "be wild!" , Trump's inaction, and invoke the existence of pardons, it becomes a somewhat disjointed, broad narrative. There are a lot of witnesses that might testify against Trump's malicious intent, as well.
Any sensible person knows Trump is responsible for Jan. 6 for an array of reasons.
But convincing a jury of undeniable intent along with, like you said, little precedence - is probably enough unease to make prosecutors question that angle.
The obstruction charge has already been used on ~300 Jan. 6 defendants and carries up to 20 years. The conspiracy charge has a lot of supporting evidence and potentially flipped witnesses. I think they might keep it narrow and stick to what they have the best chances of securing convictions on.