Becoming a registrar is a chore. In the sense, you must first obtain ICANN accreditation to become one and to do so, there is a one-time application fee of $3500 (USD). Along with that is a whole list of documents that you need to submit to tell ICANN that you’re a stable business. To prove that, you as an organization need at least $70,000 of liquid working capital.
Once registered, you must pay USD 4,000 per year to ICANN as a membership fee. I understand that for some organizations it may be too much, so only certain organizations can become registrars.
During the document submission process, ICANN applies certain checks and balances. For example, they look at certain projections for the fiscal year just to make sure your finances are on par with industry standards, that you have proper documentation under the RAA (Registrar Accreditation Agreement), etc.
All this is tedious and also long in terms of procedure. You can check the details on the ICANN website if necessary. Some players in the market approach consultants to take care of this aspect, like LogicBoxes etc.
But yeah, that’s how it happens because, if you think about it, it’s someone’s online entity that’s at stake, so ICANN has to make sure that everything is in place.