So many elements were discovered without a prize. What is special about Radium? Radioactivity is not that rare.
Before radium was isolated, the only available sources of alpha particles were uranium and thorium. The naturally occurring isotopes of uranium and thorium have half-lives of billions of years, which is on the same order of magnitude as the age of the earth, allowing them to occur in nature in significant quantities. But the long half-lives imply low rates of decay.
The Curies chemically isolated several products of the decay chains of uranium and thorium. They initially referred to them all as radium, and there were names like radium C, radium C', and radium C2 for what we would today describe as isotopes of bismuth, polonium, and thallium. Because radium 226 has a half-life of only 1620 years, its alpha radioactivity is approximately a million times more intense than that of uranium or thorium.
The availability of an intense alpha source was crucial for further experimental breakthroughs in nuclear physics. Alphas from radium were used in the 1909 discovery of the nucleus by Rutherford, and also in the 1917-1919 discovery of nuclear transmutation and the proton, also by Rutherford.