A) The amount of mana fixing in a format has a strong influence on what types of decks and strategies are playable. It is a know you can intentional turn to influence speed and archetype building in both retail limited formats and Cube. Most retail limited formats are not designed to have as much fixing as full RTR block needed to run. Furthermore, having as much mana-fixing at the quality of pain lands, check lands, fetches, etc. would also have a big impact on the speed, supported archetypes and strategies, etc. of the format.
B) If rare level lands became so common, they would be worth less and not be able to hold down the prices of the other cards in the set. If you are basically getting them for free, they can't be as effective a trading tool.