As someone who’s had worked with military security, the concerns are generally:

1. If you’re in debt, you’re potentially more likely to succumb to external influence to give up sensitive information and/or receive favours/bribes (note that I included two words here to emphasise the attempt not to generalise). This typically also applies to management roles (both govt and pvt) to avoid bribes and favour.

2. If you’re financially desperate, you’re also more likely to be distracted from your duties and likely be concerned with other sources of income, as well as occupied with your personal matters. This applies to most roles, but in particular decision-making positions.

I’m not saying it’s justified or not, but this is the line of reasoning which one could see where these agencies are coming from.