Because the oldest known cuneiform is dated to be older than the oldest known hieroglyphics. The earliest Cuneiform is dated back to around 3500 BCE while the earliest Hieroglyphics are dated to around 3400-3200 BCE. Both have some amount of proto-language going earlier but it is unclear how developed they were.

Additionally, Sumerian script has a long evolutionary history that goes back to 8000 BCE that can be traced, while hieroglyphics seem to have sprung into use comparitively suddenly. This is why some scholars say that even if hieroglyphics are a fully independent system, it is possible cuneiform still stimulated that creation of a writing system.