Radiation and Gravity (Figure 1)

Due to radiation comet starts out gassing and become fragile as a result it can break under Gravitational gradient before it enters the atmosphere, hence each fragment enter over a period of time

Most of the time It remains un affected hence enter as one unit

Earths Atmosphere

Comet has its own atmosphere hence its nuclei may not come in direct friction hence may not burst in the air

Small asteroids may evaporate or burst in the air/large rocky and metal asteroid will impact the earth