Why does my ds lite bottom screen need me to press so hard?

So, I’m not sure this is a total answer to you question, but it may help to shed some light on how these touch screens work.

The Touch screen in the DS is a “resistive” touch screen. Which is why it requires a stylus. A resistive touch screen is made up of two layers separated by a very thin layer of air. When the layers are pressed together, that is what generates the touch input.

The force which is required to activate touch could be the result of any number of things, but bear in mind the DS lite is a predecessor to the 2DS. The DS lite was out in 2006, the 2DS was a rework of the 3DS, which is a whole gen of console ahead and released in 2013. That’s 7 years of time to develop a display which feels more responsive and quicker hardware to drive it. If it feels like you’re pressing harder than you should have to, it may just be the display is worn. Imperfections in the surface of a restive touch screen are more likely to cause it to malfunction (as opposed to a capacitive touch screen which can sustain much more surface damage.