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Why was it only England that succeeded in weakening royal power?
Literal
Why England only [subject-が] royal-power [object-を] weakening [thing-こと] [in-に] succeeded [nominalizer-のか].
A historical question posing a puzzle. なぜ at the start sets up an explanatory/causal question. だけ ('only') attached to イギリス isolates England as the exception in a comparison. 王権を弱める ('weaken royal power') is an infinitive-style verb phrase; the ことに in 弱めることに is nominalized (~こと) + に (the directional/dative marker on the thing 'succeeded at'). 成功する ('to succeed') takes the thing succeeded at with に — 受験に成功する ('succeed at the exam'), 交渉に成功する ('succeed in negotiation'). The ending のか is the explanatory question form asking for the underlying reason.