Why do preheat fluorescents (in Canada and US) usually use choke ballasts over HX or CWA ballasts?
Not only US and Canada.
But the reasoning is a bit in opposite way: Series choke is used, whenever the mains voltage is sufficient as OCV for given lamp, because it is the most efficient, lightweight and so cheapest ballasting topology (the efficiency except for very low arc voltages, relative to the used mains).
As these ballast have only the mains as OCV and no means to provide heating supply other then bypassing the lamp, the starter is required to provide alternatively both the heating circuit and the ignition high voltage.
But it is limited to arc voltages in the range about 60% of the mains voltage, what mean only low power lamps in the 120V areas (so US, CA). Higher power lamps need there a step up transformer (CWA, HX,...). But when there should be already the burden with an expensive (in all: purchase cost, weight and losses) transformer, other, more comfortable systems could be made without any extra significant cost (actually this extra is frequentlly cheaper then the starter with it's socket and it offer more flexibility), so such ballasts were rather designed as RS or some even IS.
In 230V areas vast majority of used lamps have arc voltage below 100..130V, so may work with the simple series choke, so you almost do not find any other magnetic ballast then series choke preheat in the 230V areas. For some lamps were made as the RS ballasts, as these lamps suffice with the mains voltage for hot starting.
What is really inside of a magnetic rapid start ballast?
There are generally two basic forms of RS:
"120V" (not limited for 120V, but usually at least offering this mans level - mainly US and CA) contain a mains fed (auto-)transformer with two separate 3.6V windings for filaments and one for the lamp arc circuit. The 3.6V windings feed the electrode heaters (filaments) by a constant voltage, while the lamp circuit secondary is made so, it 's OCV ignite the hot lamp and it's output impedance (made either as leak transformer or series capacitor) feed the designed current into the operating lamp. As the filaments are fed by voltage, their temperature remain more-less constant (the tungsten resistance thermal coefficient help a lot here), regardless of the exact arc current. This allow to feed the lamp with virtually any current (up to the rated one) without causing the operation with too cold electrodes (so accelerated sputter), so allow e.g. powering regular, cheap 40W rated lamp on 25W only, when lower brightness is required (in homes,...) As F40T12 were replaced by F32T8 in the professional sector, more optimized 25W lamps start to appear more and more often for the home use (they save about 1W in filament heating power, are thermally optimized, so could reach higher efficacy and fall below the power limit, where is mandated higher system efficacy, out of reach for magnetic ballasts with T12 lamps)
The second system use current to the PFC capacitor to heat up the filaments, while the mains voltage ignite the lamp (European SRS).
This is accomplished by splitting the series choke ballast winding to two thinner wires. One feed the system from the mains to the lamp, second to the PFC capacitor. When the lamp is not ignited, the current flow through one winding to the filament, then through the second wire to the PFC capacitor and then through the second filament to the neutral. For this current path the magnetic field from both winding cancel out, so the ballast provide only small impedance (only small inductance due to not perfect coupling between the windings). This small inductance, together with the PFC capacitor boost the voltage a bit (by the Ferranti effect), what ease the lamp ignition.
When the lamp ignite, it form quite low impedance. Now the arc current come from two ways: Part from the capacitor (the reactive force) through one winding and second part from the mains (the real power plus some of the reactive force). Both currents are not as much off-phase, so they load both windings in very similar way as it would be single wire, almost reaching the efficiency and weight of the simple series choke, while allowing to eliminate the starter. The only costs are the need for two windings (a bit more processing cost) and a lamp sufficing with only slightly elevated mains voltage for hot ignition (so do not work on Krypton filled T8 lamps). So as the T8 ruled the fluorescent market in Europe, the SRS topology fell out of interest (on account of classic preheaters capable to operate both T12, as well as and newer krypton T8 - unlike in the US, in Europe the T8 were designed to be fully compatible with the glow starter preheat ballasts already in use)