The Dorian was traditionally the first "in order", as the other answer suggests.
The modern 7-mode system is actually an expansion of an older 4-mode system in the first millennium A.D. The 8 church modes of the octóechos system are derived from these 4, where each of the 4 has two different manners of melodizing.
There is no widely preferred term (yet) for these 4 archaic modes, but Willi Apel's book on Gregorian Chant favors the Latin term maneriae or "manerial modes". There was a traditional order, whereby they were at that time called simply by their ordinal adjectives: First, Second, Third, Fourth.
By ancient custom, each mode was defined by narrower pitch sets than an octave, particularly by their first 3 or 4 intervals. Thus, the Protos echos ("First mode"), happened to be that mode whose first three intervals were Tone-semitone-Tone (Whole-half-Whole), as we would find on D-E-F-G or A-B-C-D. The Protus mode, then, is the ancestor of today's Dorian and Aeolian modes.
By around 1000 A.D., it had become custom to alphabetize the tones of the diatonic scale, as well as the modes corresponding to those tones. The Protus mode's root tone (its "tonic") was labeled with the first letter of the Roman alphabet, and its starting interval string of Whole-half-Whole is A-B-C-D. (That interval string is also found on D-E-F-G, which is why the 11th-century medieval writers say that the First mode begins on either A or D, since they were deemed equivalent.)
The same tradition was maintained in the East, as is evidenced by the relatively recent development of the Byzantine parallage, whose tones are named after the letters of the Greek alphabet (Pa Bou Ga Dee = alpha, beta, gamma, delta). The first, Pa, corresponds to the Western La or Re (A or D), because, again, the First mode (Protos echos) was traditionally that which started with the Whole-half-Whole tetrachord.
The real question is why the First mode in order was the mode starting with Whole-half-Whole. The Greek and Roman writers between Claudius Ptolemy and Boethius might have more to say on the subject. It seems to me the tradition was carried intact through the centuries of the Roman period, and has its exordium in the Greater Perfect System of Ancient Greece, where the diatonic scale begins and ends on the note that we label as A. The theory behind that system is correlated to the tuning of stringed instruments in Ancient Greece, and so it may be that the bi-millennially-enduring primacy of order of the Aeolian/Dorian modality, starting with Whole-half-Whole, is ultimately derived from nothing more than a manner of how lyre-makers planned their string boards.
In any case, the Greater Perfect System was perceived as a composite of upside-down major tetrachords (half-Whole-Whole), not necessarily designed around the Whole-half-Whole tetrachord at all.
Given all that, I doubt the Dorian's centrality in the diatonic brightness spectrum has anything to do with its primacy. The Dorian's symmetry is, however, derivable from some tangent commentary by medieval writers on interval mirroring (flipping upside-down). Berno of Reichenau, for example, writing around the 1020s, distinguishes the Dorian's upper tetrachord species (Whole-half-Whole) for being its own mirror, but he says nothing about the upper tetrachord matching the lower tetrachord.