Q: Why are Theurophoroi so weak?
A: They carry spears.

All spear-armed infantry are weak. Theuros are less weak than many other spear-armed infantry, and even acceptable vs cavalry, but in the end they are spearmen. Weak. Losers against any decent axe- or sword-armed infantry.
They are also Greek, which rhymes with weak.
Or Hellene, which starts with hell, as in hella-weak.