I purchased the game and tried to download and unpack the zip file twice to no avail. I then installed the launcher and have been trying for 3 days to download the game and every time the download fails. I also feel that a steam key should be provided for people who have PAID for the game in beta as it shows good faith and support for a game we feel we will enjoy enough to back and it would be nice to get that same support in return.
Payed for a broken and unusable product
Are you sure you have the latest version of the itch.io client? You need to have at least 18.7.0 for the unpacker to work properly. Also check that your anti-virus is not blocking or otherwise altering the download.
Note that the availability of Steam keys is addressed in their FAQ.
The developers have various reasons why Steam keys are not provided at this time. You should wait until the game is released on Steam, if having a Steam version is something you desire.