Jadyn, I know precisely the type of bread you're referring to and the reason for the disparity is down to whether or not they contain fat and/or oils.
Much of the supermarket bread made by Hovis and their franchise bakeries (e.g Family Loaf), don't contain any fat or oils (Hovis for many a year have produced both white and brown bread). Consequently, the bread is drier than some other mass produced loaves making it more appealing for toasting regardless of the thickness of the bread. In fact, some other manufacturers exclude oil deliberately in their "Toastie" bread ranges to alter the texture for this very purpose.
Morrisons also tend not to include oils and fats in their own brand bread ranges(made by Rathbones, which Morrison's own) and other suppliers. Roberts Bakeries use fats and oils with some discretion whilst Braces use only rapeseed oil.
As far as the oil/fats are concerned, rapeseed oil was formerly the most common oil in bread, but due to it's increasing cost on the commodity market, manufacturers are substituting palm oil. Palm oil is one of the highest saturated oils in existence and has gained dominance because it is hugely cheaper than rapeseed oil. Until recently, palm oil's primary use was lubricating industrial machinery. Please don't be misled about so-called "sustainable palm oil" on packaging as there is no legal definition of the term and the ecosystems out in Borneo and other islands are being systematically destroyed in order to grow the palm species required for the oil.
The widespread use of palm oil is one of the reasons why successive governments no longer bleat about the harm we are doing to ourselves in eating saturated fat. They have been unable to curb it's widespread use as manufacturers say that reverting to the healthier rapeseed oil will increase the cost of processed food massively.