This is because, once you are beginning using the panel to add streams and create user accounts, you do not realise about HTTPS. Then when you have remembered it is impossible to go back and change the http protocol, there are two reasons of why you can't do this:
1 - All the channels, movie's, etc, logos you must change it to HTTPS. This might sounds easy if you think to use a SQL command to change the http to https, but if you were adding photos which were not saved in your server, then some photos which are stored in a third party will not be available if the owner has not installed https certificate.
2 - The users who are using your IPTV service have to update your IPTV app ("you must change the API requests to https") and m3u8 list ("if a user tries to connect to a stream that the has http, the player will not play the stream until he update the m3u8 list).