Because at the end of the day, child restraint manufacturers are looking to clear a profit. They are interested in child safety but no one manufacturer is likely to make any substantial changes that will send the average Joe running for a competitor's product.
Think about it, the average parent uses an infant bucket for a year, then buys a cute, cozy looking convertible and converts it forward facing.
If he discovers that the Graco My Ride has a 30 pound forward facing limit, he'll just by the Evenflo Triumph 65 at the same price point with a 20 pound forward facing minimum.
Unfortunately for real change to be made, it would have to be a standard/law/requirement/policy from above.
I'm hesitant to base anything on weight limits because we all know how varied children would be, but I would be perfectly comfortable with AGE limits. For example, an age minimum of 3 on any harnessed seat in forward facing mode would bypass the issue of weight minimums entirely, ensure that all children under three are rear facing, and force manufacturers to discontinue or redesign seats that won't function to the third birthday rear facing, like the Comfortsport.