I can only guess, obviously, but maybe you're offering an hourly rate of pay which is insufficiently greater than the amount you pay commercial staff to make this an attractive proposition to someone who is required to drive to take the position on, this assumption based on the fact that those who have accepted the position have then elected not to start employment with you.
The H&S, insurance and administrative requirements which are a strict necessity of commercial cleaning are routinely ignored in domestic cleaning and householders are generally likely to compare providers on a cost basis; the result of that, in my opinion, is that a cleaner who can drive is likely to be able to undercut your company in the provision of domestic cleaning services while earning a better hourly rate. Those staff who have attended interview may have done so for no other reason than market research or may have realised subsequently that the travelling time allowance which you offered was insufficient to achieve a reasonable overall hourly rate.
I don't love jobcentres or gumtree as staff sources and, in your position, would be more likely to place a small ad in the local paper or, and this would also be a reluctant choice, get an agency to find someone for me.
My first business was set up with domestic cleaning identified as the most lucrative market; three months after starting trading I abandoned it completely. With that in mind, someone better qualified might be able to offer better advice.
Incidentally, I tried to use the phrase' my' and 'assumption' in the first sentence and the site won't let me; that's as strict an application of the 'never assume' rule as I've ever seen..