Muhamed wanted to get laid, therefore adoption is Haram.
Short story: Muhamed fancied the wife of his adoptive son. So Quran 33-4 was revealed a short while after, which banned adoption and allowed marrying adoptive son's divorcee.
Long story: Step 01: Making adoption forbidden "That is but your saying with your mouths. meaning: `your adoption of him is just words, and it does not mean that he is really your son,"
Step 02 Permitting the prophet to marry his adoptive son's wife (wasn't after their divorce as some erroneous translations suggest, he divorced her so that the prophet could marry her after): "(So that (in future) there may be no difficulty to the believers in respect of the wives of their adopted sons when the latter have no desire to keep them) (33:37)
And that's how millions of orphans will not find a happy home: because one man wanted to shag 1400 years ago.