Well for a few reasons
A -- Attucks name is variously recorded in various documents
B-- Nubian Sq is a name given to a historic square by people currently living in it -- no guarantee that one hundred years from now that Nubian will mean anything to the people residing in the area
The original name Dudley honored a family with a complex relationship to Boston -- no doubt about that -- but they were of and for Boston and hence like Attucks a worthy candidate for a name for an important square
There is no evidence at all -- that ancient Nubia or any of its descendants -- have any connection to Boston in any way -- except through the art and artifacts excavated by the MFA / Harvard archeological project and which is now housed at the MFA
By that measure Kenmore Sq should be renamed Giza Square for the similar artifacts at the MFA excavated from the Giza plateau
Note that if you think that Roxbury and Dudley / Nubian Sq will always be the way it currently is [it was once quite a Central / Eastern European Jewish enclave] -- you haven't studied the history of Boston neighborhood evolution very thoroughly
Paul Revere's house and environs have been at one time or another -- in rough chronological order:
Puritan English
mixed "Yuppy" English [well Revere was from a French Huguenot background]
Irish
Eastern / Central European Jewish
Italian
Mixed Yuppy
etc., etc.
And finally -- in this very strident, vituperative and confrontational era -- we need some common ground and reference points that everyone can agree upon:
Attucks is recognized by Americans for his role in igniting the American Revolution -- he and Joseph Warren are two of just the handful of names of martyrs to the American Revolution who are known to more than historians
[the others being the names of grade schools in Lexington memorializing those killed on the Battle Green April 19, 1775]
He has a simple memorial on the Common -- indeed the first memorial in Boston -- but he deserves more
and the time is ripe as we are near to [indeed already beginning the celebration of the 250th of the lead-up events such as recently the Boston Massacre and soon the Tea Party] celebrating the 250th anniversary of the American War of Independence [aka the Revolutionary War]