(This is my answer to a question posted on Quora)
Mechanical Turk is a platform for work. Workers get paid, which makes now Amazon a payment processor. Payment processors are moving money on behalf of other people, and therefore are under heavy scrutiny from the US government for issues related to money laundering (AML), counter-terrorism, tax compliance, etc.
One of the key things that is required from financial institutions is to have a “Customer Identification Program” (CIP), also known as “Know Your Customer” (KYC) process. The CIP/KYC is a set of procedures that the financial institution needs to follow to establish that they know the true identity of a customer. The processes that each financial institutions follows vary, and the exact processes are rarely available to the public, as they are considered security measures. Furthermore, the practices are regularly monitored by regulators (OCC, Fed, FinCEN, etc) and change over time to follow best practices.
In your particular case, the most likely reason is that Amazon was not able to verify your identity.
If you are in the US, Amazon most probably can get your SSN and other personal details and verify whether you are a real person. However, even if you live in the US, if you have no credit history, no bank accounts, and so on, the verification will come back with low confidence. Following standard risk management processes, Amazon could plausibly reject such applications, as part of their CIP processes: it is better to have a false negative (rejecting a normal account) than having a false positive (e.g., accepting an account that will be involved in money laundering or tax-evasion schemes).
For other countries, the ability of Amazon, to follow CIP/KYC processes that conform to the US regulations, varies. I assume, for example, that the cooperation of US with UK or Australian authorities is much smoother compared to, say, Chinese authorities. So, if you live outside the US, the probability of having your account approved depends on how robust is the ability of Amazon to verify individual identities in your country.
Given that Amazon gets paid by requesters, I assume their focus is to establish CIP processes first in regions where potential requesters reside, which is not always the place where workers reside. This also means that you are more likely to be approved if you first register as a requester (assuming this is an option for you), and then try to create the worker account.