The Pentium became Intel's budget processor when the Core architecture came out. When the i7s, i5s, and i3s are tested and binned, the dies that are still fully functional but didn't quite muster i3 levels (only 1 or 2 cores work, some of the cache is defective, only works at lower power levels/clock speeds, etc) are binned as Pentiums.

The leftovers that aren't completely scrap are stamped as Celerons and Atoms.

I've grossly oversimplified it, but that's the general idea.