Because the central registry can set whatever price they wish to set and whatever price the market can/will bear. It might make sense to lower the price and thereby invite more registrations but . . . different countries/entities think differently.
.br.com is just a clever use of a 2 letter domain that involves selling subdomains of that .com domain, using the idea that the 2 letter dot com subdomains can (pretend to) be "like the real ccTLD". It has a certain conceptual appeal, at least until you start to wonder about who your neighbors are on the domain, how their SEO efforts might affect your efforts, whether the "whole of the domain", including subdomains, might be penalized, etc.
I've never bought one. Never will. I'm my view pseudo ccTLDs are just that -> pretenders to the actual country's domain. What happens when SearchEngineX decides that the root domain has become nothing but a SEO play or something else less than desirable? What about governance? A ccTLD has a certain level of governace. A subdomain is subject to contract or TOS and, as most people know, TOS are easily rewritten.
Other firms are now attempting to sell subdomains of keyword domains, a la city.typeoflawyer.com and the like. There was a time when subdomains were a ranking ploy that worked. Lately . . . I haven't seen much evidence of the ploy working lately. Maybe next Wednesday @ 3:00 it will?
what's the differnces between .com.br and .br.com ?
.com.br is part of the .br Country Code TLD for Brazil. .br.com is not. Google and the other search engines would consider a site on .com.br to be Brazilian or related to Brazil. A site on the other would not benefit from this as it is just another .com site.