Why can't I keep any leaves on my Dieffenbachia?

Not enough humidity..

Needs indirect lighting and more humidity.

https://www.gardeningknowhow.com/houseplants/dumb-cane/growing-dumbcane-dieffenbachia.htm

These plants do not like to be moved. Dont over water

If you want to start again, I have had great success airlayering a diffenbacchia and creating two plants out of one!

wer: you are over watering the poor plant. Water only once a week and don' t flood it.

These are tough plants and I'm sure it will be fine but, first, check underneath the leaves for any signs of bugs such as aphids, etc. - if any are present, visit your local nursery and ask for their advice re what insecticide to purchase. If possible, take an infected leaf (with the bugs) in a plastic bag then they can identify the type.

Next is watering. It's important to water well (to encourage healthy root development) then let the soil almost dry out. Watering "well" directs the plant's roots to grow downward, seeking water. "Watering well" means taking a plant to the kitchen sink and watering until the water comes out the drainage holes. You then wait for dripping to stop (this avoids root rot) THEN put the plant back in its outer decorative pot assuming you have one, of course.

Now, naturally you can't do this with your large plant, therefore, a plant in a pot that size, if it were mine, I'd pour in 5 to 6 cups of water BUT then leave it until the soil is almost dry.

Use common sense, around the 7th day (after watering), stick your finger in the soil about 2" - if it feels damp, wait a few days then check again. You will soon find a schedule as to what the plant needs re watering. It might need water once a week or ten days or more - it depends on the dryness in your home, sun exposure, etc.

(Watering often and with small amounts is incorrect and much harder on a plant; it can contribute to fungal problems, mildew, etc.)

Re light, if your plant is a fair distance from a window, I'd move it to a much brighter location. Avoid really intense sun as it will bleach the leaves but it needs bright light.

One more thing ... check the soil to see if there's tiny brown/black insects (they look like flies) crawling over the surface. If you see any, take the plant outside, dispose of all that soil, wash out the pot with hot sudsy water then repot your plant with new potting soil. Adding a few handfuls of Vermiculite is wise as it helps the soil. It's cheap and usually found at Walmart, nurseries, and sometimes, dollar stores.

If you find in time your plant looks too leggy, simply saw through the trunk then put the sawed-off part in a jug of water. When it has sufficient roots, you could pot it in the same container as the original plant (to give a more bushy look) or repot it in a separate pot. I hope this info helps.

Sounds as if the plant is in the wrong environment... e.g over or under-watering, bad drainage thru a poor potting mix, wrong location ( too much sun ? )... or wrong temperature. Many nurseries sell well presented stock from an ideal environment.... but that is not the environment in our home. Try moving it to another location and only water it if your poked finger tells you that the Dieffenbachia needs a drink. The dead margins of the leaves may be suggesting a mandatory change in its environment. The future prunings should strike in a jar of shallow water placed on a shelf in indirect and warm light.