I suggest you contact the freelancers that declined the offer and ask them for honest opinions on what made them decide to not partner up. FYI, 4 candidates isn't a lot in my opinion. Did you advertise the job on LinkedIn or other platforms?

SuperVGA said:

10000 lines isn't much, if it's organised well, like I also think you're saying.

Exactly, that's a small codebase that shouldn't scare off any competent developer.

Maybe one of your assumptions is incorrect. Are you sure the code is decent quality? Did any of the freelancers see it before declining? Perhaps you could reach out to someone to do a quick review of the codebase.

Are you certain money isn't the issue? Freelancer sites are notorious for people that are, lets just say “very junior”, spamming low bids on jobs.