One way in which protists are considered primitive is that they are mostly unicellular, and the ones that are multicellular do not organize their cells into specialized tissues. However, protists have been evolving just as long as other life on Earth and are not necessarily primitive beings.
Scientists originally thought protists were the most primitive organisms alive, and their forms had remained stable throughout the billions of years life has existed on Earth. It is now known that the similarities between protists are actually superficial, and they are actually many different groups of creatures that have been changing and adapting for billions of years. Some protists are derived from more complex ancestors. Therefore, their seemingly primitive organization may not be representative of what truly primitive life forms are like.
Protists are defined not in terms of their supposed primitiveness but according to an inability to place them comfortably within the plant, animal or fungal kingdoms. Many scientists do not recognize the category at all as biologically valid, because protists are so diverse and have multiple origins.