Islander_fan said: "The main difference is that telecharge has a designated website for that but Ticketmaster doesn’t."
This is true, but I believe you are looking at it backwards. Ticketmaster often has an area for a promo code to be entered on every page, even when there may not be any active discounts available. When people see this, though, if often prompts them to google whether there is a discount available so they can enter the code to save $$$.
Sine Telecharge was started by theater owners, they didn't want their ticketing system to trigger that behavior, so they only accept discounts on a parallel website that you either know about in advance or find once you already have a discount code.