The Cosmic Microwave Background (CMB) is the residual radiation from the early stages of the universe, specifically from a time known as the "recombination epoch." This epoch occurred approximately 380,000 years after the Big Bang when the universe had expanded and cooled enough for neutral atoms to form.
The CMB is still observable today because of the expansion of the universe. When the CMB was first emitted, it was in the form of extremely energetic and hot photons. However, as the universe expanded, these photons underwent a process called "cosmic redshift." The expansion of space stretches the wavelength of the photons, effectively reducing their energy.
Due to this redshift, the high-energy radiation of the early universe has been stretched to longer wavelengths, specifically into the microwave region of the electromagnetic spectrum. This redshifting of the CMB photons is the reason why we observe it as microwave radiation today.
Furthermore, the expansion of the universe means that the CMB photons have been traveling through space for billions of years, encountering less dense regions as the universe expands. These photons have effectively been "streaming" towards us since their release during the recombination epoch.
While the CMB photons have been traveling for a vast amount of time, the universe itself has also been expanding during this period. As a result, the distance between the original source of the CMB photons and us has increased significantly. This expansion, along with the redshifting, allows the CMB to still be observable today.
In summary, the CMB radiation is still reaching us because the expansion of the universe has stretched its wavelength and the photons have been traveling through space for billions of years.