I've tried producing cleaning products many many years ago and asked the same question, for me, it turns out that the error rate of having a bottom cap where the cap might not be screwed in properly / defect in the plastics, can be quite a headache, I mean it's only a very small error rate but when it does happen it always made an disproportionate mess. So unless I have a more advanced manufacturing facilities, I decided to stick with a top cap but made it flat so that if the user desired they can be placed upside down.