What invention caused scientists to stop using the five kingdom classification system?

Because the five-kingdom model was always a terrible, terrible loose association based only on the superficial traits observable through optical microscopes. “Protist” was a catch-all category for all single-celled organisms with nuclei. Its three main categories — plant, animal, and fungus — aren’t even accurate genetic descriptions of all human-consumable species; kelp and most species of seaweed (like the nori in sushi or the indigenous sea vegetables of the British Isles) are more closely related to species of old “protists” than to each other or to land plants. Members of “monera” were so radically different from each other at the chemical and genetic level that the differences between the other kingdoms weren’t even comparable.

Imagine if you had to classify wheeled vehicles. Plants are like Ferraris, animals are like Aston Martins, fungi are like Porsches. “Protista” was like a category consisting of all SUVs and 4x4s, and “Monera” was like a category including Lamborghinis, Mustangs, Toyota sedans, motor homes, wheelbarrows, and the model-T.

It would be ridiculous for a wheeled-vehicle classification system to pretend that a Ferrari and an Aston Martin are more different from each other than a Lamborghini and a wheelbarrow. That’s what the old five-kingdom model was doing. So we redid the system.

it's diluted or concentrated

in a diluted solution, you can add a lot more solute but in concentrated only a little more can be added