It's not known for sure, but it may be a surface area to muscle volume ratio problem related to energy and oxygen delivery. Once muscle fibers get too big, it may be too energetically costly to build more protein and maintain the fiber as it is, and there may be something telling the fiber that it's no longer economical to build. For example, when muscle fibers get too large, they can start having a problem with inorganic phosphate accumulation which can affect contractile function. Additionally, mitochondrial density tends to drop, capillary density per fiber tends to be maintained, but per unit of cross-sectional area tends to decrease. (Source: Stronger by Science Episode 20: Reading Research, the Placebo Diet, Muscle Memory, and Alex Kolliari-Turner)