I respectfully disagree. The reason he did not, IMHO, is the same as the reason the prohibition of intoxication and usury was gradual: Breaking a bad, but strong habit cannot be done by a decree! How many people succeeded in quitting smoking cold-turkey? After Lincoln freed the slaves by decree, the bloodiest war in US history was launched and he was assassinated!
Compare that approach to this teaching of the Quran,
"Should he not then conquer the obstacle? And what may make you understand what the obstacle is? Untying of a neck! (freeing a slave)" (90:11-13)
Asalaamu Alaikum,
That is another good point. I am just saying that, if Muhammad(pbuh) stopped slavery there and then, alot of people would've suffered because of it. In times of wars, there are orphans, widows and people who have nothing or no-one to care for themselves, these people would likely end up dead or become the slave of the Polythiest and we all know how they treated slaves. By still allowing slavery, the Prophet(pbuh) put in strict rules in their treatment, many of these slaves would later be freed and during their time under slavery would be cared for and treated in a just manner.
The Prophet(pbuh) used to say that marrying your daughter to a Muslim slave is far better then marrying your daughter to any Non-Muslim. The first person to do the Athaan was a former slave, Bilaal (may allah be pleased with him).
There's such a deep research on this, it's quite fascinating when you have an in-depth understanding of slavery during the time of the Prophet(pbuh). I don't have this though, just the basic concept of why Slavery was around at the time and how Islam was as an antidote a gradual breaking of slavery.
edit: woops, I completly missed the main point :S I also meant to say was that if people were captured by Polythiests as slaves, they would later be forced to fight against the Muslims which would make it more difficult.