Because it's essentially just Quicken Spell, except that it is patently worse (since you are limited to getting the SAME spell twice) or completely broken (when used with things like Quicken Spell and Spell Perfection).
All around it was badly designed power creep. Paizo has already implied that it doesn't ever intend to allow more than two spells in a round. It just cheapens what we already have (Quicken Spell).
That, and it probably isn't open content, and would thus create legal problems.
No "probably" about it-- it's not open content. "Twin Spell" is intellectual property of Hasbro (i.e. Wizards of the Coast).
Nothing preventing you from converting it yourself for your home game.
You can't copyright game rules. It is perfectly legal to make your version of this that does the same thing described in your own words.