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On a quick note, another fugue - which is very famous, actually - which uses the sequence of tonic - subdominant -tonic etc. in the exposition is the fugue in Bach's G Minor Sonata for Solo Violin. While it's a fugue in a totally different league (after all, true polyphony on a violin is pretty much impossible) it is an example of a subject that strictly requires being answered in the subdominant. But even in the Well-Tempered Clavier, there are no fugues of this model, so the G Minor Sonata is somewhat an odd exception.
Furthermore, the question of why the sequence tonic - dominant - tonic actually establishes the tonic better than other sequences is because from a harmonic point of view, the dominant is the only scale degree leading to the tonic, and thereby creates a 'cadence' in the tonic. A perfect cadence is much more firm than a plagal one, as is easily heard by trying both out on a piano. Therefore, while I'm sure other sequences can work, they were not preferred by former fugue composers as tonality and harmony always came before polyphony, and the key had to be firmly established somehow.
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