We split seasons that are twice the size, simply because we don't allow for as much lead time before airing the first episode of the season as the BBC does. The split is causing BBC America headaches (and money for ads to get the viewers back come spring); it's not making anything more appealing...
The BBC in general is causing BBC America headaches.
The BBC's inability to decide when "Asylum of the Daleks" was going to air made it almost impossible for BBC America to promote the season before it launched -- they couldn't produce posters, ads, commercials, etc., that had a specific date.
Specifically on the split seasons, BBC America doesn't have the budget for promoting both halves.
Also, on split seasons on American television in general...
We had a discussion at work about split seasons, and we were trying to figure out who to blame. (My coworkers were upset at the midseason break for The Walking Dead
.) The thing is, no one's to blame. American television has always
had split seasons, it's just that we never advertised them as such. Shows usually take a breather in the schedules during November and December. The networks use that as a time to put specials and other events on the schedule, while the programs can catch up productive-wise and build up another bank of episodes. It was always an informal thing, but shows like Battlestar
formalized the broadcast break into a narrative break.