The way the sequence works isn't from guns to knives but from guns to Improvised Explosive Devices. A 50 pound sack of common fertilizer is far more lethal than knives and assault rifles combined.
One reason why explosives have become the weapon of choice of middle-east terrorist is because its cheaper and easier to acquire than firearms. Surprise: firearms are either illegal or strictly controlled in most middle-east countries. Explosives are available from your nearest garden store, have multiple delivery methods and require very little training, even when delivered via suicide bombers. If no volunteers, they just strap it to a kid.
Also, IEDs are easier for cowards. Remember, OKC bomber Tim Mcviegh was apprehended by chance. Had it not been for a highway patrolman noting his auto had no license plate he may have escaped and done it again, and again.