Why does instant coffee have a poor reputation? Among coffee drinkers instant coffee is generally considered as a last resort and many avoid it entirely, even though instant coffee is often cheaper per cup and doesn't require specialized coffee brewing equipment.
Because the taste and aroma are horrible. I've been places (for instance Shanghai and Beijing) where it's hard to get a cup of coffee outside of business areas catering to foreigners. 

The instant coffee is terrible. It is not a pleasure to drink. It's only palatable (to me) with a ton of evaporated milk and a lot of sugar. For someone  who usually drinks coffee black this ruins the coffee experience.

To answer your question: for people who love the smell and taste of good, freshly roasted, freshly ground coffee - instant coffee simply doesn't compare. Hence the poor reputation.