A normal battery charger for car use consists of a transformer to reduce the voltage to an acceptable level, a rectifier to turn this voltage into DC, and a meter to let you know that current is flowing. Current is "regulated" by the impedance of the transformer windings. For a car battery, this current is OK at about 4 amps, and the system will self-regulate to an extent. Using the same charger on an SLA which will normally be much smaller than a car battery will cause the battery to fully charge very rapidly. It is more than likely that over-charging will occur, and the battery will give off gasses, hydrogen and oxygen. A car battery with screw tops can be topped up, an SLA lacks this ability, so an over charged one is expensive ballast. Charging any Lead acid battery at too high a rate can cause the internal plates to warp. As there is not much space between the plates, this can lead to internal shorts, which is a recipe for disaster.
So SLAs need a charger that supplies the appropriate voltage at a current that is safe for the battery.
NiCads and NiMHs have different chemistry and internal construction. Voltage supply is not important as long as there is enough, but current supply is, and needs to be regulated down to a safe level. Again, overcharging damages the battery by boiling off the irreplaceable electrolyte.
The different families of battery give different clues to indicate a full charge to the charger, so the charger needs to have the right kind of sensing circuit, hence the differing types of charger.
As a general rule, long and slow is better than short and fast. Not just with batteries, come to think of it.