People have learned to be more trusting of something labelled as a specific medicine when they want guaranteed results, something that they can buy from a pharmacy and particularly when they control the dosage and can see what they're putting into their drink, hence slightly opaque water-like liquid, or powdered mix or effervescent tablets might be ideal.
The more the branding is marketed as a medicine you can still buy off the shelf the more people will focus on its medical efficacy instead of its taste or other placebo effects, which is the goal here.
The mild taste of a lot of pills is subtly reassuring as it's not associated with food but with treatment and relief, which is what someone is looking for when they're feeling awful like that.
Ideally there shouldn't be a flavour when coming out so there's nothing to associate it with the act of vomiting.
Simple instructions such as "drink in small sips over the course of 30+ minutes" and easy details on how much water and how often per day can also be comforting to the person when they are in that situation just wanting to be taken care of and not make any extra calculations.
Also someone buying it in a hurry whether for themselves or for someone else might look for whatever looks like the safest and clearest instructions, grants maximum relief/healing and is least likely to have contraindications that they can purchase ASAP to alleviate the suffering, so if it can be bought off the shelf or requested at the counter that's a win for ease of access!