I swear that about 5 to 6 years ago, I didn't know a single woman (or child) named Ella. I mean, sure there was Ella Fitzgerald, but I can't hardly say I knew her. Times have changed though, and I can count more than a dozen little girls named Ella.
I think part of the rise in popularity of Ella is that it was a name riding on the coattails of the immensely popular names Emma and Emily. Ella became a less common an alternative to those names, and so happened to find her own increased popularity because of it.
World-wide, the name Ella is popular in several other countries, and not just English speaking one. England/Wales, Canada, Australia, Belgium, Denmark, Sweden, Ireland, and Scotland currently feature the name Ella on their top 100 name lists.