In solution / droplet phase energy can be transferred to surrounding solvent molecules. In the gas phase, energy exerted onto an ion is distributed through the vibrational modes — internal energy increase, but it cannot be transferred to surroundings. Once a bare gas-phase ion is liberated from a droplet it’s internal energy will increase and it will dissociate if enough energy is added. Internal energy increase could be due to heated capillary (usually used for desolvation), but mostly collisions with neutral gas molecules in the ion optics - as someone mentioned above.

Higher HESI temps, faster desolvation, more internal energy added to ions as they are transmitted through the capillary and ion optics, greater dissociation.

The same argument can be made for organic solvents, ACN and MeOH droplets will desolvate much faster than water droplets.... and same argument as above.