Why was it a better idea to use ground up beatles to make red food colourings and not just red fruits...?
The reason that ground-up beetles make for a better food dye than, say, red fruits, is that the former is natural; whereas the latter is not.  [1][3]  While natural dyes such as those made from beet juice, paprika, and carmine are not always as safe as they sound [2], cochineal extract, or carmine dye, is generally safe and has been used for centuries to color foods like sausages, pies, and dried shrimp. [1][3]  Even so, cochineal extract can trigger anaphylactic shock, asthma, or a dangerously abrupt drop in blood pressure, so those with allergies should take note. [2]
There are two main reasons why foodmakers would use ground up bugs to make food dyes, rather than just using red fruits. First, insects are a cheaper source of red pigment. For example, cochineal insects are native to Latin America where they live on cacti, and are now mainly farmed in Peru [3]. According to the BBC, millions of these tiny insects are harvested every year to produce carmine, one of the most widely used red food colorings in the world [3]. Second, when insects are dried and smashed, they give way to a vibrant long-lasting red dye [4]. Carmine was even used by the Mixtec Indians of pre-Hispanic Mexico to color their clothing and show off their social status [6]. By the 1900s, Americans had begun using cochineal dye to color sausages, pies, dried shrimp, candy and jams [6].    