Well, as I have recently advertised a role on Mumsnet Jobs with the exact term I can categorically say it is NOT because it's a poor salary.
We are happy to pay market rate for the role, and we have excellent holidays, very generous pension, free medical coverage, flexible working, gym subsidy, childcare vouchers.....
The reason we don't put the salary is because jobs here are graded. People know the salaries linked to the grades if they are internal but we don't want to put an actual salary as we tend to keep salaries unpublicised. There are other internal people who have similar roles who may want to apply, they are welcome to come and see me and ask about the salary, and external candidates can email and ask. Those going through an agent will be told the range.
But it's not because it's a low salary. To some extent, for this role, we don't really know what to pay so we need to test the market and see what applicants we get and what they are looking for.
It's a shame if that puts people off and would suggest people should rethink that strategy.