You cannot just create a .zip and then change the extension. The zip must be packed in a specific manner. Probably the resulting .epub is not correct. That mobipocket and apparently kindle simulator (ironic, since the Kindle doesn't support ePUB...) can open it, is something else.
Try the program ePubPack (somewhere here on the forum) or assemble the ePUB with Sigil. That is much easier and will deliver a structurally good ePUB. Additional bonus, Sigil has a built-in validation.