The first thing to remember about Einstein's Nobel Prize is that it was given for "his service to Theoretical Physics, and especially for the discovery of the law of the photoelectric effect" So it was not strictly given to him for the photoelectric effect. But to understand why that theory was highlighted above others, it is important to consider the timeline.

Einstein published the photoelectric effect in 1905. along with several other papers, including Relativity and Brownian motion. At the time there was great interest in Black Body radiation, various spectra from excited atoms and many other phenomenon that we would now consider quantum mechanics. But at the time the complete framework was not known and these things were very mysterious. We know see the photoelectric effect as mathematically simple compared to relativity, but for many years after 1905, very little actual progress was made in what would later become quantum theory.

Rutherford scattering was not discovered until 1911 and the Bohr model of the atom was not published until 1913, and we know it was not a complete theory of the structure of the atom. That's six to eight years of little progress, where the only known thing about quantum physics was Einstein's work. Throughout all of the quantum revolution, Einstein's work served as a 'north star' for other developments. It would take until 1926 for Schrodinger's equation to be published, which is five years after Einstein's prize was awarded. So yes, Einstein's work on the photoelectric effect was very important and opened up many avenues of inquiry that ultimately gave us the quantum revolution of the 1920s and beyond. At the time, it was considered extremely important. While Relativity was also important, it didnt have any serious applications until the splitting of the atom in 1938.

Edit: I am not a professional historian, but I do have a PhD in physics.