Simple - the more 'things' you have in your tank of discus (particularly as a beginner) - e.g. décor, driftwood, other tank-mates, and in particular - PLANTS - the more difficult it is, and becomes, to keep the tank squeaky clean, and free of any unwelcome spots for wastes & debris to collect, amass, and fester, thereby providing a multitude of breeding grounds for the development of undesirable bacteria, among other harmful pathogens.
This environment (e.g. a planted set-up) creates a situation where ideal water quality and conditions are very difficult to achieve with any degree of consistency, especially where large, daily infusions of fresh water seldom receive priority attention.
This is why the highly recommended approach to starting into discus-keeping is a bare-bottom tank - obviously so extremely easy to keep sparkling clean with very little effort - that's the secret to discus-keeping success !
Hope you get the picture.
And Apologies Rolla, for neglecting to welcome you to Simply - Here's hoping you learn, enjoy, and succeed at keeping discus.