Experiment using hydroxides of alkaline earth metals
I would attribute this two 2 reasons:

Reason 1: Insolubility of Magnesium hydroxide.

Magnesium hydroxide is almost insoluble in water and will not easily dissolve in water.

Reason 2: Safety is one of the reason of the choice of compounds.

The formation of hydroxides of alkaline earth metals are generally exothermic and this intensity increases down this group.


Reaction of magnesium oxide with water will produce the
corresponding hydroxide:

$$\ce{MgO + H2O → Mg(OH)2  (very slight reaction)}$$

(The reaction quickly stops because the magnesium hydroxide formed is almost insoluble in water and forms a barrier on the magnesium preventing further reaction).
The reaction of calcium oxide to produce its hydroxide is
exothermic:


$$\ce{CaO (s) + H2O (l) ⇌ Ca(OH)2 (aq) (ΔH_r = −63.7 kJ/mol of CaO)}$$


Barium oxide is  harmful to human skin and may cause irritation on exposure for prolonged periods or in large quantity.


Its reaction with water is also know to be violent.



So in my opinion it is much better to user hydroxides of Ba and Ca.