Like if a put a toast with jam on top in the microwave and warm it up altogether, jam feels a lot hotter than bread. Why?
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There are two thing going on in this question:
First, microwaves have a frequency that is close to the natural harmonic frequency of water so they are really good at heating things that have lots of water in them but bad at heating dry foods. The ELI5 version is to consider pushing someone on a swing. The swing has a natural frequency that it “wants” to swing at. If you time your pushes right and push at the same frequency, the swing will go higher. But if you just push randomly or at the wrong frequency, the swing won’t work very well: sometimes you will be pushing against the swing and slowing it down.
Second, all substances have a property called “specific heat capacity” which determines how its temperature changes when you add or remove energy. You have to add a lot of energy to water to raise its temperature but lead only needs a little energy. If you put a kilogram of lead and a kilogram of water in the same oven, the lead would get hot faster.
Going back to the swing analogy, if the person on the swing is heavier, you will have to add more energy by pushing longer or harder to get them up to the same height as a lighter person. Also, the swing could rotate or go side to side so some of the energy of your push might go to movement that does not increase the height of the swing. In a real substance, the configuration of its molecules determines how it can vibrate, rotate, or wiggle in ways that don’t contribute to temperature.
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