I would understand if this gets deleted, but the thing I wish to know is why Africans were more prone to forced to go to USA as slaves than South Asian people which was if not more, equally colonized as Africa...
Hugh Thomas' 'The Slave Trade' is a comprehensive history on the subject that is worth exploring.
There is one overwhelming reason why Africa was preferred over elsewhere: location. The first major European slavers - the Portuguese and Spanish of the 15th Century - had colonies in both Africa and the Americas. Labour was required in the American colonies, primarily in mining, sugar and tobacco. Why travel half-way around the world to seek slaves in today's southern Asia?
It is often forgotten that the overwhelming majority of African slaves were traded from local rulers for a variety of European goods ranging from textiles to firearms: it was remarkably rare for Europeans to seize slaves directly, and such action was contrary to the many treaties made with African kings and chiefs. This is the basis of the notorious 'triangle':
Travel to India not only took considerably longer, but was far, far riskier. India was also colonised much later. Prior to the British East India Company's victory at the Battle of Plassey in 1757, India was effectively ruled by the same native princes and Mughal Emperors who had done so for centuries. Even after Plassey, it took several decades for British Rule to be formally established. By this point, the abolitionist movement was in full swing.
It's also worth noting that Europeans at the time appear to have respected India as an ancient civilisation that simply needed some 'help' in re-establishing its greatness; there was a regard given that was rarely encountered in Africa. Whilst it's true that African kings and princes were entertained in Europe, it seems this was viewed more as an exotic curiosity, rather than as respect for a 1,000 year civilisation.
A more pertinent question is why Europeans didn't rely on local, Amerindian slaves?
Initially, they did. However, several issues, real and imagined, dissuaded them from this ready source of labour.
Perhaps the most real fear was that native American slaves would very easily be able to escape and incite rebellions, calling in assistance from other tribes and peoples. On the contrary, African slaves were as lost in the Americas as Europeans, and were even ocassionally armed and mustered into rudimentary militias to fight Amerindian raiding parties, before being put back to work.
Another frequently cited reason for not using native American slaves was that they were simply not tough enough; they succumbed to the harsh work too easily. African slaves were perceived as being of much hardier stock, able to withstand punishing work in the intense Caribbean heat. What probably contributed to this view was that some of the first native American slaves were put to work in South American gold and silver mines. These mines were notoriously brutal, more so even than the later plantations, and had a fearsome rate of attrition, so much so that slave holders decided that their local supply of slave labour was simply not up to the job.
I once asked Gareth Austin, one of the great scholars of West African economic history, why it was that Africa was so plagued by slavery compared with other regions. The answer he gave was centred around labour productivity, and he has written papers that at least touch on the broad question, such as 2008's "Resources, techniques, and strategies south of the Sahara."
West Africa has historically had quite low labour productivity, but relatively high wages (i.e. the bargaining power of free workers was high.) The land was not, in general, very productive, with thin topsoils, lack of easily navigable rivers, low economies of scale, difficult disease environment, few domesticated animals (that would survive), and long distance from other markets. Paradoxically, it was also an area where land was abundant, and thus labour's opportunity costs were surprisingly high. There were no masses of local labour to exploit because they had no alternative, as everyone free to choose had an alternative: simply go somewhere else and farm.
As Austin writes: "Principals wanting to increase labour inputs beyond those available from themselves and their families faced what might be called the ‘Nieboer’ problem: that with land available to all, and in the absence of a technology yielding economic advantages of scale, there was no wage which it would be profitable both for a prospective employer to pay and for a prospective worker to accept, as opposed to working for him‐ or herself."
Thus, forms of labour coercion were already built into the economy, as control over labour was already a precondition for producing much beyond subsistence. But of course, these were scaled up enormously in response to European demand for slaves, and anyone making the argument that the transatlantic slave trade is just some sideshow to domestic African slavery is simply wrong.
This situation was obviously not the case in the densely populated and tightly stratified society of Mughal India, where local workers had few enough exit options, and thus, could be employed in place without having to ship them halfway around the world. Employers did not face the same difficulty extracting rents from their workers, because unlike in Africa, those workers seldom had anywhere else they could go and make a better living.
This is not to say you never see slaves taken elsewhere in the world, or even that you never see slaves brought to Africa - the "Cape Malay" community in South Africa have their origins in slaves brought by the Dutch East India Company to work at the Cape of Good Hope. But that is a very different part of Africa, and a very different set of circumstances.
This is also not to say that, once slavery had been effectively abolished, that exploiting workers with very poor outside options from low-wage countries stopped. Semi-coercive migration from relatively low wage countries (including India, though also Ireland) in the form of indenture was widespread. Once it became more profitable for employers to offer indenture and pay for passage to the destination country than to use other arrangements (wage labour: expensive, slavery: banned), the practice became widespread, and approximately two million labourers from India went out into the world under this system, leading to large Indian-heritage populations in the Caribbean, among other places. The logic is similar to slavery: a market is made because the indentured labourer is cheaper and easier to control than a free labourer, but the (potential) indentured labourer cannot leave their home area without resources they don't have. And so, employers trade the upfront cost of transportation in exchange for control and only having to pay low wages at the destination. The magnitude of Indian migration through indenture is smaller than the transatlantic slave trade, but still very large.