= Solution
In the Condon approximation the intensity from $v''=0$ to $v'$ contains the <Franck-Condon factor>
$$
|\langle\chi^A_{v'}|\chi^X_0\rangle|^2.
$$
The lower ground-state vibrational <wavefunction> is localized near its equilibrium bond length. If the two equilibrium lengths are similar, it overlaps strongly with the upper $v'=0$ <wavefunction>, but its overlap with highly excited upper vibrational states is usually small or suffers oscillatory cancellation. If the equilibrium lengths differ enough that a vertical excitation reaches the outer portion of the upper well, high-$v'$ transitions can be strong while the $0\to0$ overlap is small. Thus \b[the two ends of a long progression are not normally both intense]. This is an overlap argument, not a ban on either set of transitions; anharmonicity, a varying electronic transition moment and unusual <wavefunctions> can modify the intensity envelope.
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