Answer by taking a quote from the following article:

The Spice Girls are an English pop girl group formed in 1994. The group originally consisted of Melanie Brown ("Scary Spice"), Melanie Chisholm ("Sporty Spice"), Emma Bunton ("Baby Spice"), Geri Halliwell ("Ginger Spice"), and Victoria Beckham, nee Adams ("Posh Spice"). They were signed to Virgin Records and released their debut single "Wannabe" in 1996, which hit number one in 37 countries and established them as a global phenomenon. Their debut album Spice sold more than 31 million copies worldwide, becoming the best-selling album by a female group in history.

The phrase "girl power" put a name to a social phenomenon, but the slogan was met with mixed reactions. The phrase was a label for the particular facet of post classical neo-feminist empowerment embraced by the band: that a sensual, feminine appearance and equality between the sexes need not be mutually exclusive. This concept was by no means original in the pop world: both Madonna and Bananarama had employed similar outlooks. The phrase itself had also appeared in a few songs by British girl groups and bands since at least 1987; most notably, it was the name of British pop duo Shampoo's 1996 single and album, later credited by Halliwell as the inspiration for the Spice Girls' mantra.  However, it was not until the emergence of the Spice Girls in 1996 with "Wannabe", that the concept of "girl power" exploded onto the common consciousness. The phrase was regularly uttered by all five members--although most closely associated with Halliwell--and was often delivered with a peace sign. The slogan also featured on official Spice Girls merchandise and on some of the outfits the group members wore. The Spice Girls' version was distinctive. Its message of empowerment appealed to young girls, adolescents and adult women, and it emphasised the importance of strong and loyal friendship among females.  In all, the focused, consistent presentation of "girl power" formed the centrepiece of their appeal as a band. Some commentators credit the Spice Girls with reinvigorating mainstream feminism--popularized as "girl power"--in the 1990s, with their mantra serving as a gateway to feminism for their young fans. On the other hand, some critics dismissed it as no more than a shallow marketing tactic, while others took issue with the emphasis on physical appearance, concerned about the potential impact on self-conscious and/or impressionable youngsters. Regardless, the phrase became a cultural phenomenon, adopted as the mantra for millions of girls and even making it into the Oxford English Dictionary. In summation of the concept, author Ryan Dawson said, "The Spice Girls changed British culture enough for Girl Power to now seem completely unremarkable."  The Spice Girls' debut single "Wannabe" has been hailed as an "iconic girl power anthem". In 2016, the United Nations' Global Goals "#WhatIReallyReallyWant" campaign filmed a global remake of the original music video for "Wannabe" to highlight gender inequality issues faced by women across the world. The video, which was launched on YouTube and ran in movie theatres internationally, featured British girl group M.O, Canadian "viral sensation" Taylor Hatala, Nigerian-British singer Seyi Shay and Bollywood actress Jacqueline Fernandez lip-syncing to the song in various locations around the world. In response to the remake, Beckham said, "How fabulous is it that after 20 years the legacy of the Spice Girls' girl power is being used to encourage and empower a whole new generation?"  At the 43rd People's Choice Awards in January 2017, American actress Blake Lively dedicated her "Favorite Dramatic Movie Actress" award to "girl power" in her acceptance speech, and credited the Spice Girls, saying: "What was so neat about them was that they're all so distinctly different, and they were women, and they owned who they were, and that was my first introduction into girl power."

how did "girl power" become popular?
The phrase itself had also appeared in a few songs by British girl groups and bands since at least 1987;