The characters were all conveniently located in one place. If you forgot who someone was, you could easily flip back and reference it. In many of Christie's books, she even thoughtfully provided a cast of characters with the name and a brief explanation of their role in the novel.
With too many books these days, I lose track of characters. I've even gotten close to the end of a book, read a character name and gone "Who the heck is that?!". Either the author created too many people for me to keep up with, the characters' names sound too much alike, or...let's be honest...I've been interrupted so many times when reading that I can't remember who's who.
I can't help but think there are other readers like me out there--scatterbrained folks who love a good read but don't want to keep up with too many names. Maybe they're moms or other busy people who just want to keep it simple. I can usually read just 20 minutes at a time before I'm running up to take laundry out of the dryer, dashing out the door for a carpool, falling asleep after a long day, or hearing "the doctor will see you now."
I'm making a conscious effort to limit the number of murder suspects for my books. Five is really a good number--not so many that you lose track of them, but not so few that you don't have any sense of surprise when the killer is revealed.