Many factors contribute to applicant flow. For example, the job type (e.g. brain surgeon, retail clerk, lawyer), the nature of the job (e.g. straight commission, six-figure base), and the company/location/benefits all act as attractors or detractors, thereby affecting applicant flow.
Additionally, this online hiring system was designed to repel unqualified job seekers and attract those who meet your specific qualifying criteria. Job boards generally collect resumes, thereby inviting 'quantity', not necessarily 'quality'. Although you may have fewer applicants, those applicants responded to every qualifying question and specific requirement.
Craigslist can be a great source for applicant flow. Be sure to use the Instant Ad feature that allows you to easily post your job on Craigslist. Craigslist is the only job board that does not accept automatic feeds, so you must manually post your ad.