There's nothing convenient about it. In a nuclear explosion there are trillions of neutrons flying outwards from the epicenter. Through simple statistical probability some will collide with something. Only other radioactive elements would be unstable enough to be fully destroyed by the energy of being slammed into by a neutron. Thus neutrons get stuck inside the nucleus of the atom.

However, without more protons to keep the forces inside the atom balanced it must be released. So the atoms themselves will become radioactive just naturally trying to relieve that imbalance.

Here are some of the radionuclides (atomic isotopes) made by a nuclear explosion.

https://en.m.wikipedia.org/wiki/Activation_product

Neutrons attach to atomic nuclei and transform them into other elements by literally slamming into the nucleus. It's not really about convenience, even a 0.00001% chance of a neutron hitting an atom just right to transform it is inevitable when we talk about the number of atoms in a given volume. The molar mass for atmospheric oxygen (2 oxygen atoms) is about 32 grams. 6 x 10^23 molecules of O2 weighs 32 grams. There's a lot of atoms around a nuclear explosion taking place on earth.

Any atom can be radioactive, they do not need to be heavy. Tritium for example is a radioactive form of hydrogen with 2 neutrons. Hydrogen is stable and not radioactive with 1 or 2 neutrons. There's generally a ratio between neutrons and protons (really simplifying this) where elements tend to be stable. Go outside of that ratio and the element tends to be unstable and will decay into something else. The why is related to quantum mechanics.