Can someone tell me why?

Organoman nailed it. no info no real help. some theories. The pics show yellowing and that's common with nitrogen issues but also a lot of other things like overwatering and leaching/underfeeding sometimes also PH or lockout. Good Luck!

Did you noticed any insects on your plants? Last picture looks like there is something going on at your leaves. Check both sides of them. Their larvaes might be munching your roots wich could lead reduced nutrient uptake and this partly discoloration if my guess was right

It looks like a Nitrogen and Calcium deficiency, but more info or a diary would definitely help. Thia is like walking into the doctor and saying you feel bad, what is it? You need to describe a few things to make life easier to give a helpful answer.