Spider-Girl is set in the MC2 Universe, a "What If" world where the daughter of Peter and Mary Jane Parker did not die (as told in regular continuity), but instead survived to become a hero in her own right. The MC2 Universe originally consisted of nearly a half-dozen titles, but declining interest left only Spider-Girl remaining, backed by a devoted an particularly vocal fan-base who have kept the comic going in spite of several threatened cancellations.
Part of the popularity might be explained by the incredibly sustained creative team. Writer Tom DeFalco has scripted nearly ever issue, and the artistic team has also seen very few changes indeed. The teen-ager targeted storyline isn't everybody's cup of tea, and since it is out of the regular universe then many cash-strapped Spidey fans clearly opt to survive without this book. But still, it's worth a look at any rate.