Why can't some light bulbs be used with timers?
Timers come in two varieties:


Timers with a neutral wire.  These are connected between the hot and neutral wires of the circuit, parallel to the load.  They're powered like any other device, and work by switching the hot (and possibly neutral) wires of the controlled circuit.  These timers can control any device, but require a location with access to both the hot and neutral wires.
Timers without a neutral wire.  These are connected in series with the load, and work by restricting the current through the hot wire of the controlled circuit.  Because they use the load as part of their own power circuit, they can only control resistive loads (essentially, light bulbs and some electric heaters), but can be installed anywhere (in particular, as a drop-in replacement for a light switch where only the hot wire passes through the switch box).


LED and fluorescent bulbs (and motors, and...) don't work properly with the second type of timer, either because they require a sufficient current to let any electricity flow, or because too low a current will damage them.