Well, here's my admittedly biased two cents:
Necromancy has a powerful fantasy and mythological tradition outside the context of D&D. Decades before Gary Gygax ever thought, "hey, wouldn't a wargame be cool if you could fight trolls and dragons?", fantasy authors like Robert E. Howard and Lord Dunsany were using the terms "necromancy" and "necromancers" in their work. Even Tolkien used it (remember Sauron's time spent undercover, known simply as "The Necromancer"?). There's just so much more inspirational source material than there is for something much more generic like transmutation.
For all that players often like to choose schools like evocation, evocation doesn't mean what it means in D&D anywhere but D&D. "Boom spell mages," who are pretty much unheard of in most fantasy, literary and mythical traditions, are usually "fire mages" or the like when they appear. Elementalists have more of a life outside the D&D context than evokers do. (My version of Microsoft Word doesn't even recognize "evoker" as a word.)
The division of spell schools in D&D has always been on a very technical level; that's why you have "conjuration" instead of "summoning." This makes for more accurate rulespeak, but at the cost of some of the flavor that draws people on more than a technical level. It's probably no coincidence that the PS2 features games like "Summoner 2" instead of "Conjurer 2"; "summoner" sounds better. It's more limiting, but more appealing to the wider audience. (See also "transmutation" and "alchemy"; the latter is more limited, but much more widely known.)
This may seem like a silly concern when you figure that most of the D&D book-buying audience doesn't worry as much about that wider appeal, but let's face it; store managers who don't necessarily play D&D may be more interested in stocking titles that sound "classic fantasy." Newbies to the hobby might be drawn in by the idea of playing a necromancer in ways that an abjurer wouldn't hook them. There are advantages to supporting the schools that have strong visual images already built-in, instead of trying to base your book on the rules strengths of a school and hoping that will be enough to sell it. And of all those schools, necromancy has the strongest visual language, with maybe the exception of enchantment (and even then, that's based more on classic legendary enchantresses like Morgan la Fey than legendary enchanters like... uh... well, I guess you take my meaning).
Besides, necromancy is the school of choice for players and DMs who have tons of skeleton and zombie miniatures. That's what helped me make my decision.
