Why isn't ionization energy called cationization energy?
Atoms can have electrons added to them or removed from them, which in turn ionises the atoms. However, electron addition and removal aren't the same thing in terms of enthalpy changes. The key here is the difference between the 
process and the enthalpy changes during the process - as much as an atom is ionised during the production of a cation or an anion, the enthalpy changes are different. Because of this, there are two different terms used for the enthalpy changes during these processes:


'Ionisation energy' is the enthalpy change when one mole of gaseous atoms form one mole of gaseous positive ions (cations), under standard conditions,
'Electron affinity' is the enthalpy change when one mole of gaseous atoms form one mole of gaseous negative ions (anions), under standard conditions.


As seen from the definition for 'ionisation energy', it's already described that there's a production of cations, so it would be redundant to call it 'cationisaion energy'.  The word 'ionisation' in itself is typically used to describe the removal of electrons from an atom, which could also contribute to the term's name.



The reason why there are two different terms is due to how the electrons are dealt with during ionisation. The removal of electrons is always endothermic, whereas the addition of electrons is only sometimes endothermic.

When removing electrons, more energy is required than what's released to remove the electrons from the atom, hence why it's always endothermic. This is due to the Coulombic attraction between the negatively charged electron(s) in the atom's orbit and the positively charged proton(s) in the atom's nucleus. The required energy will increase with each successive removal, since the electrons will get closer to the nucleus and the attraction will increase because of it. This enthalpy change is explicitly called 'ionisation energy'.

When adding electrons, only the first addition is exothermic whilst the rest are endothermic. The first addition is 'easy'; you're just adding an electron to a neutrally charged atom, and the energy released during this process is greater than the energy required. However, with each successive addition, the energy required will increase. This is due to the Coulombic repulsion between the anion and the electrons, so more energy is required than what's released to 'force' the electrons into the atom's orbit, hence why successive electron additions are endothermic. This enthalpy change is explicitly called 'electron affinity'.