Okay, I wrote my undergradute dissertation on a similar topic so I feel myself qualified to give this one a go.

The idea of an anti-slave trade bill had been on the minds of legislators on both sides of the Atlantic for a while before 1807. The US had outlined its intention to end the importation of slaves within 20 years in 1778. The idea was supported by both Northern and Southern states. Northern states were quicker to outlaw the practice of slavery. Especially in Pennsylvania were a large Quaker presence had long since been wrapped up in anti-slavery campaigning. This came largely from a mixture of moral repulsion toward slavery. But also because the systems of agriculture in the north didn't favour the maintenance of a large slave population. Meanwhile landholders in Southern states realized the end of the slave trade represented an economic opportunity. In contrast to other slave states were mortality rates were incredibly high. The USA was able to stabilize its own slave population. This meant slave owners were in posession of a powerful commodity and the ending of an importing in slaves essentailly further empowered and enriched those who already possessed them. All this meant congress was able to reach a consensus on the banning of the slave trade. Not out of any real disgust with the trade (although many would have felt that way) but with the realization of an economic opportunity. The story in Britain is quite different ,and more to my expertise, so I might go a little more into depth.

There had been enormous popular support for anti-slavery in Britain since the 1780s. For example, in 1780 a petition for the ending of the slave trade in Manchester was signed by 1/5th of the population! Despite the public outcry and several attempts Parliament failed to pass a bill until 1807. Historains have interpreted this in a variety of ways.

The first to take a real critical look at the matter was Eric Williams in 'Slavery and Captial' (1945). He argued, that abolition was not the result of Christain benevolance but of economic imperatives. Slavery and the slave trade were no longer profitable or economically viable, so Parliament felt itself able to legislate against it and then claim that legislation as a moral victory. This interpretation has been challenged quite a lot. Seymour Dresher in 'Econocide' (1977) dispels this quite successfully. He shows the slave colonies were still enormously profitable up to 1807. In fact, at that time, they were the most profitable wing of the British Empire. Why then, in 1807 does Parliament choose to commit econocide and abolish the trade, but maintain slavery itself?

The answer, in my opinion, is geopolitical. Firstly, let's note that anti-slavery campaigners certianly felt that the end of the slave trade was just the beginning. Much like a modern human rights organization they had aims. And ending the trade was their first one. They then intended for the end of slavery to come and end pretty quickly afterward. However, there initial campaign was focused on the trade. They were able to illustrate, quite effectively, the horrors and sheer inhumanity of the slave trade. There campaign was firmly focused on the trade and its impacts in Africa. The actual slavery was happening in the Carribean and they didn't yet focus their campaign on this. The plan was to end the trade then the act itself. There was also this perception that the slave trade was somehow 'closer to home' than slavery itself. It involved British ships and British sailors. Often anti-slavery argued that the act of slave trading was severly psychologicaly damaging British sailors. Mental I know.

Anyway, this put a lot of pressure on Parliament to do something. But it still didn't for almost 30 years. Mostly this is geopolitical. In the 1790s the French had abolished slavery and the slave trade as part of their revolutionary constitution. The British establishment were terrified of liberal France and were not about to imitate their politics. However by 1807 Napolean was in charge and he'd firmly put slavery back on the menu. He saw it as an economic necessity to get France back to making money through slavery. This gave the British the oppurtunity to then support anti-slavery meanwhile also claiming a moral victory over Napolean. Furthermore, in 1807 Britain felt itself hegemonic in the Atlantic theatre. After Trafalger in 1805 all other naval threats were gone. Its Carribean colonies were secure from external threat and it really did rule the waves. This confidence probably went some way to letting Parliament believe itself capable of abolishing slavery and setting a global agenda. The end of the trade also gave the Navy a self imposed power of seizure on suspected slavers of the African coast.

Finally and probably most importantly, the British were terrified of slave insurrection. There had been a number of serious slave revolts in Jamaica not to mention Haiti were a successful slave state had decimated a British invasion force. Decimated is innaccurate even. Almost half of the invading force were killed. It was supposed that by ending the trade, and the influx of slaves. The population might be easier to manage, in imitation of the United States the British set out to stabilize its slave population. It was also supposed that an end to the trade would result in slave owners caring for their slaves better therefore lessening the threat of revolt. Seen as they couldn't rely on buying new labour every time a slave died.

In conclusion I don't think either state abolished the trade from a moral standpoint. I think that was certianly a factor but not the primary one. In both cases, it would be fair to say, the system of slavery, and the profits it produced, were so firmly entrenched in the establishment that its speedy abolition seemed impossible. Meanwhile the abolition of the slave trade had numerous potential benefits both in providing economic oppurtunity, securing slavery's future and offering piecemeal reform to those who so strongly opposed slavery.