The early ADB implementations were not hot-pluggable, as first of all the software driver stack only discovered the ADB peripheral devices at boot time, and hardware was implemented in a way that could lead into damage to devices.
The largest hardware change that could have required proper hot-plug support could possibly have been Macintosh Portable. On this device, the integrated keyboard also communicates via ADB, as does the trackball. However, according to user manual, it also required to be shut down from the menu before connecting ADB peripherals, otherwise all ADB devices may lose their addresses.
So, the main reason may not actually be the hardware, but software/driver support for re-enumerating newly inserted devices. But there is also little point making the driver support for hot-plugging, as it should be discouraged due to lack of reliable hardware support.
The hardware for the ADB connector is also very simple. Comparing the ADB port schematics with for example the non-ADB keyboard port, the ADB port seems to have very little effort made to protect against potential issues from hot-plug events.
Early ADB implementations had a fuse protecting the 5V supply pin, which many people have complained to blow when reconnecting the ADB cable. Possible reason for this might be the inrush current to charge the capacitors on the peripheral power supply pins. Later implementations removed the fuse so at least a blowing fuse stopped being a problem.
One source of problems is the electromechanical interface, i.e. the Mini-DIN connector. The connector does not guarantee any order in which the pins make a connection.
If the unplugged device gets charged with static electricity, and the ADB data pin happens to mate first when plugging, it will discharge almost directly via the ADB controller data IO pin as there is very little protection from ESD events.
The connector pins for ground, ADB data and 5V supply are mapped so that when inserting the connector at an angle, the 5 supply and ADB data might be the first two pins that mate and the ground pin mates last. In the event that some other device pulls the ADB data pin low while one device has only +5V and ADB data connected, it puts excess stress to the ADB interface driver and the unpowered device, as basically the unpowered device may try to suck power from the ADB data pin (CMOS chips have built-in protection diodes that conduct when IO pins go above supply or below ground).
So what was most likely done to finally allow hot-plugging ADB devices (on PowerBook G3 "Wallstreet" maybe as it boasts hot-plug support) is adding enough protection against surge currents and ESD on power supply and data pins so the interface can sustain the obvious hazards of hot-plugging.
And of course, handling the detection and enumeration of freshly plugged devices in the driver software, and their removal.
Just as a side note, compared to ADB, hot-pluggable interfaces such as USB actually define these things, such as in which order the connector pins must mate, and how large surges are allowed on hotplug, etc, so it needs slightly more effort than just wiring logic signals and power between devices through a random connector.