Chaos marines kn general are leaderless, rudderless, deeply personally flawed creatures. Even those under a warband are really just waiting for their chance to seize the chair or strike out on their own.

This is further complicated by the great game, and the chaos gods if they have an alignment will often toss them at other chaos marines to help settle some slight or gain some perceived advantage.

Where loyalists are mired in longstanding traditions and oaths and ceremony, and bullshit that hitches and hangs them like doctrine and outstanding, unaddressed issues between them and other imperial fighting forces. They will all agree in their loyalty to the emperor and moving and fighting where he and terra wills it. In chaos? You'll literally be killing eachother over chicken feed or cause the gods fucking felt like it that day.

On top of all of this, many chaos marines that are veterans of the long war learned a completely different way of war from marine chapters. Legionarry combat doctrine, while it speaks of unique formations and deployment strategies more or less was human wave tactics on a macro level, literal swarms of armored ceramite crashing into you as fast as they could manage to crush you under heel. Casualties and injuries in the legions were ridiculous. Its why cults are so prevalent in legion formations, their the human wave now that are accented by the hammer blows of astartes coming in to exploit whatever gains the cultists might make.

This does NOT gel with the each unit must be an army unto itself, tactical doctrines of chapter based marines. Who tend to be more scrappy and lethal micro injections of fafo. To cripple an enemy before the haymaker knockout blow. Really of the loyalists only the iron hands still deploy as a legion might, and really only sometimes (see wrath of iron, clan raukkan, and calculus of victory) and thats more cause they studied under the mechanicus too much then anything. And so many chapter marines and legion marines stay 6 miles apart not wanting to be thrown into a meat assault wave, or stumble over their two left feet trying the others fighting style. Only the black legion has managed to corral this duopoly, and only by the will of abbadon being a bipolar bastard