Because it is not gonna happen. electrons move in specific shells with specific speed. just compare this to this question why did you fall when you are on a scooter or cycle at any 2 wheeler when it is in rest and you lift your feet from ground why didn't you fall when you are in motion with 2 wheeler. this is because at rest only one force is acting gravitational which attracts you. but when you are moving 2 forces are acting gravitational force and force of you due to which you are moving. now moving force lessens the effect of gravity. That's same with the electrons of atoms they re in motion that's why they don't fall into nucleus. same thing is with solar system.
hope this helped. if you didn't understood something then post a comment and I'll talk to you.
Yeah u r ryt , first Rutherford also thought like that & he also gave his postulate on this that electron will fall into the nucleus but this theory disguarded because it nothing happen like that , actually electron doesnt fall into the nucleus because though they are attracting each other , electrostatic force of attraction but at the same time an equal & opposite centrifugal force is acting on it which balances the force , Thats y it does not