franicum is a naturally unstable element
Francium is extremly rare, as it is highly radioactive and the most stable isotope has a half-life of 22 minutes. This means that if a francium atom is formed it will certainly disappear very very quickly. At any one time there is only about 25g of naturally occurring francium in the whole earth. Its exreme rarity means it is very difficult to find. There are no uses for francium because it does not last long.
Francium is a natural chemical element but the total quantity of francium in the earth crust is only approx. 30 g. Francium has ca. 40 isotopes and isomers but only two are natural: 221Fr (in the neptunium decay chain series) and 223Fr (in the actinium decay chain series). Artificially preparation of francium isotopes is also extremely difficult and expensive; and the chemistry and physics of isotopes was developed essentially after 1940. Supplementary, the most stable isotope of francium has a half life of only 21,8 minutes and is strongly radioactive; the half lives of the artificially prepared isotopes are more smaller.
This is sufficient reason to explain why francium was later discovered and why even today is only slightly studied.