The usual explanation I see given is that Venus is, as Regardie put it, "the occult planet which represents the Genius of this Order." Of course, he was a member of the Stella Matuina (Latin for "Morning Star," a title given to Venus, along with Evening Star), so Venus was certainly strongly applicable there, but the link is somewhat more tenuous when considering the original order and its other offshoots, all of which employed references to the Sun in their name. "Golden Dawn" is obvious enough, while "Morgenrothe" is "Dawn" (from the German for "Morning Redness") and AO is, according to Mathers, the ancient Egyptian root for "Dawn." Venus, therefore, is not as important in the name, at least.
Venus does have significant occult value, however, that gives it a prominence above the other planets when looked at from a certain perspective. Some of this is covered by Westcott:
"[The Chief Adept] has Mars and Geburah at his right hand, and Jupiter and Gedulah at his left hand. He faces Venus in the West, the Evening Star, which represents the entry of the Candidate who has toiled all day until the evening. At even he enters the Western door of the planet Venus, that sole planet unto whose symbol alone all the Sephiroth are conformed. ... [The newly admitted Adept] enters through the Green side of the Vault. Green is the colour of growth; let him see that he grows."
The idea of growth is equally important, although it could be argued that this is countered by the strong symbolism of death in the 5=6 ceremony itself. The colour green is interesting, however, as it is the flashing colour of red, and these two colours are employed on the lamen of the Hierophant, the Outer Order officer applicable to the grade of 5=6.
There is another occult significance, however. The Hebrew letter to which the planet Venus is attributed to is Daleth, which means "Door." It makes sense then that the Venus wall would be the door to the Vault.
However, the most important reason is because the previous Sephirah that the initiate has come from is Netzach, from the 4=7 grade, which is attributed to the planet Venus. Therefore in order to enter Tiphareth, wherein lies the Vault, the initiate must enter through the Venus door.