Since the operation of this latching design is built around the voltage dependent capacitance of the base-collector junction in the transistor I had a quick peek at the "varicap"-entry in Wikipedia where the following is written:
 

Substitutes for varicap diodes

All semiconductor junction devices exhibit the effect, so they can be used as varicaps, but their characteristics will not be controlled and can vary widely between batches.

Popular makeshift varicaps include LEDs,[5] 1N400X series rectifier diodes,[6] Schottky rectifiers and various transistors used with their collector-base junctions reverse biased,[7] particularly the 2N2222 and BC547.[clarification needed] Reverse biasing the emitter-base junctions of transistors also is quite effective as long as the AC amplitude remains small. Maximum reverse bias voltage is usually between 5 and 7 Volts, before the avalanche process starts conducting. Higher-current devices with greater junction area tend to possess higher capacitance. 

I wonder if the reason for the 2n2222/bc547 simply is that they are just jellybean parts that people already have at hand and is just using it because of that.  Not that they have any particulary good (or in most circuits - bad) BC capacitance and voltage dependency of it.  It's not like those two transistors can be classified as higher-current devices....