Apart from the perfectly valid dramatic reasons to do this, you can explain it from a cosmic or external perspective because:
(a) the other Lightbringers are with Orlanth in the underworld; and
(b) the Thunder Brothers are either children of Orlanth (so derive their power from him) or are ascpects or parts of Orlanth (so derive their power from him).
Note that Flesh Man is a Lightbringer, so, in your Glorantha, Heortling common magic suffers as well!
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