Big registrars can’t afford any support costs since they prefer to squeeze the price down as far as possible, and therefore they prefer to simply lose or outright drop any customer in case of any and all problems. Conversely, small registrars may charge more, but have better (i.e. actually existing, and sometimes even dedicated and personal) support for when things go wrong, and have a vested interest in keeping you as a customer.
A small registrar might also be so small as to know you personally, which will help monumentally against any social engineering attacks.
Full disclosure: I work at such a registrar, but you’re probably not in our target market.