Why H/Cs/I atomic clocks are more complicated than Rb? It is known that low-cost commercial Rb atomic clocks are fairly simple in construction: gas cell is probed by a light source and excited by RF around 6.8Ghz. When frequency is right - we see decrease of the photodiode signal.
But if we look at Cs and H clocks - they are not using this principle, but instead have fountains/masers, complicated mechanisms for selection of right states... Why Cs/H clocks can't use same simple design with RF excitation of gas cell and probing with light source?
Same for Iodine - it has huge number of lines in visible and NIR spectrum which are easy to probe at low cost. Aren't there any that can react to RF excitation <12Ghz and can be used as a time source?
 A: According to Rubidium Frequency Standard Primer,

Before the availability of spectroscopic lasers, rubidium was the atom
of choice for gas cell devices because of its unique ability to use
isotopic hyperfine filtering of the pumping light from a Rb lamp.
The efficiency of the optical pumping process is enhanced by a
fortuitous overlap between the optical absorption lines of the two
naturally-occurring isotopes, 85Rb and 87Rb. This is the main reason
that rubidium is used in most (non-laser pumped) gas cell atomic
frequency standards. Rubidium is unique in that the 85Rb isotope can
serve as a hyperfine filter to remove one of the hyperfine components
from the light emitted by an 87Rb spectral lamp as shown in Figure 6.
In this cartoon figure (see Figure 7 for real spectra), the top plot
shows the two hyperfine components in the optical spectrum of one of
the 87Rb D-lines. Because both hyperfine components are present at
equal intensities, this light would be ineffective for the purpose of
optical pumping. The middle plot shows the absorption spectrum of 85Rb
gas. Notice that the “a” and “A” components overlap while the “b” and
“B” components do not. If the 87Rb lamp spectrum is passed through the
85Rb filter cell, the resulting filtered spectrum is shown in the
bottom plot. The desired “b” component is now significantly larger
than the “a” component, making the filtered spectrum more effective as
an optical pumping source for an 87Rb absorption cell. The filter cell
can be separate or integrated with the absorption (resonance) cell by
using natural Rb with both isotopes.

