Clarifications of types of Gybes:
Step Gybe: Change feet before rig flip or at very start of rig flip.
Strap to Strap (STS): Change feet after start of rig flip. (sometimes after boom in hands again - sometimes during rig flip just as boom come back into hands) Not necessarily putting the front foot in front strap first on new tack, but this is where the name comes from. Timing pretty much the same whichever foot goes in first.
Step Gybe has some big advantages in rough, bouncy water on a slalom board, especially with a large rig that is harder to 'float' around in the flip. And also in marginal planning conditions where staying on the plane through the gybe and on exit is unlikely.
STS Gybe can be faster in nicely powered slalom conditions on
flattish water where the speed in is quite high, the rig is 'weightless' in the flip and the exit speed is quite high and fully planing. In this situation, because the board is going relatively fast, the tail does not tend to sink and bog in, and the sailor can stay both feet between the front and back straps. By the time the feet are switched, the rig is back in the hands and powered. Because the board is still planning, the sailor can keep both feet behind the front straps and immediately power up off the fin. (this is why I almost always put the back foot in first in this situation, as I already have weight on the rig and mast foot, and can keep it driving though the new front foot between the straps as I put the back foot in. In rough and bouncy water, the advantage goes back to the step gybe and there is more control during the rig flip. In STS, the board can get very bouncy in that time of rig flip, foot switch, especially if speed is bleeding off very quickly with bouncing.
One major key to a good STS gybe is putting the back foot well forward on the carving rail on entry. This stops the board bogging down as it slows and helps the sailor keep the weight projecting forward. This somewhat alleviates the often seen need of Step Gybers to step right forward (front foot well ahead of the new front strap almost to mast foot) to try the flatten the board out again the maintain planing. Some boards don't like to carve in as easily if you do this (foot well forward on carving rail). It depends a lot on the plan shape, and to a lesser extent on the rail shape. Larger, wider tailed slalom boards are often harder to do this on (110L and up) as they have straighter plan shape with wider tail. More traditional rounded pintail boards and speed boards usually do this well.
On my very narrow, 40cm speed board, my carving foot is touching the front strap, and I sometimes sail away with both feet still in both front straps for a second or two if I don't plane out fast.
So to sum up, IMHO, the reason it is somewhat favoured for fast Alphas, is that these Gybes tend to be done at very high speed on flat water.
The reason why the vast majority of Slalom racers use a Step Gybe is that they are usually in bouncy rough water and using large heavy rigs, somewhat overpowered. The emphasis is more towards 'Safe' than 'Fast', because it is very slow if you blow it.