With knuckle push-ups having the two additional benefits of building up wrist strength and knuckle toughness compared to the convention push up, why aren’t knuckle push-ups considered the “standard” push up?
Knuckle push ups don't ''build wrist strength'' they just teach you how to stabilize better your push up, palm push ups be it with fingers pointing forward or backward or with palms up all build actual wrist strength and produce small forearm hypertrophy through isometric stretch while knuckle push ups don't.
also doing push ups on your knuckles elevates you slightly making the actual push up even easier, try doing normal weighted push ups and weighted knuckle push ups, you will be able to push more weight with the knuckle variation.
knuckle push ups are never better or harder, they just require more stabilization and more pain tolerance.
If you want to strengthen your knuckles then push ups are a sub-par exercise for this goal, something better would be actually punching hard things with progression.
Start by punching buckets of water,then sand then wood then walls and so on to create micro fractures in your bones and make them grow thicker and denser.
Knuckle push ups are only good if your goal is to do finger push ups, you start doing them on knuckles then on all fingers and then slowly use less and less fingers until you can do 1 finger push ups. Other than that they don't serve any other purpose and are mostly useless.
If your goal is to get stronger wrist and big forearms then direct wrist work is needed; pronations,supinations,extensions,flexions,adductions,abductions and isometric holds. Training the wrist can be done with your own body weight,hammers,dumbbells,your own other hand,tables,chairs,ropes,rings,bars,barbells,bands and even the weight of other people. As long as each workout is harder than the previous one you will get strength and muscle gains regardless of the method used.
If your maximum is under 50 pushups, do 200 a day. If your maximum is above 75, do 300 pushups a day. Repeat the ODD/EVEN routine for a total of 10 days. Then take three days off and do NO upper body pushing exercises that work the chest, triceps, and shoulders.Your calorie burn rate depends on your weight. For example, a 155-pound person can burn 563 calories in an hour running 5mph. If you weigh 150 pounds, 10 minutes of moderate pushups burn 57 calories, while 10 minutes of vigorous pushups burn 96 calories.