Arashi (Lan , lit. Storm) is a Japanese boy band consisting of five members formed under the Johnny & Associates talent agency. The members are Ohno Satoshi, Sakurai Sho, Aiba Masaki, Ninomiya Kazunari, and Matsumoto Jun. Arashi officially formed on September 15, 1999, in Honolulu, Hawaii, and made their debut CD on November 3, 1999. The group was initially signed to Pony Canyon and released one studio album and six singles--beginning with their 1999 eponymous debut single before moving to the Johnny's subsidiary label J Storm in 2001, which was initially set up for their succeeding releases.

Arashi began 2002 by releasing their first single under J Storm. "A Day in Our Life" was used as the theme song for Sakurai's comedy drama Kisarazu Cat's Eye and debuted atop of the Oricon weekly singles chart selling 226,480 copies. Their subsequent releases of the year, "Nice na Kokoroiki" and "Pikanchi", were used as the theme songs for the anime Kochira Katsushika-ku Kameari Koen-mae Hashutsujo and Arashi's movie Pikanchi Life is Hard Dakedo Happy (pikanchi Life is HarddakedoHappy, Pikanchi Life is Hard But Happy) respectively.  Starting in 2003, the group would only release two singles every year until 2007. Both singles released in 2003, "Tomadoi Nagara" and "Hadashi no Mirai/Kotoba Yori Taisetsu na Mono", debuted at number two on the charts. The song "Kotoba Yori Taisetsu na Mono" marked the first time a member contributed to the penning of a single, as many of the group's songs are produced by other lyricists, composers, or musicians. Sakurai, who is the designated rapper of Arashi, has since written all the incorporated rap lyrics of the group's releases.  2004 was Arashi's fifth anniversary since debut. They began the year by releasing their twelfth single, "Pikanchi Double", in February as the theme song for their second movie Pikanchi Life is Hard Dakara Happy (pikanchi Life is HarddakaraHappy, Pikanchi Life is Hard Thus Happy). Selling 89,106 copies in the first week, it is Arashi's lowest-selling single to date. In celebration of their anniversary, the group released a greatest hits album 5x5 The Best Selection of 2002-2004, which included their second 2004 single "Hitomi no Naka no Galaxy/Hero".  In 2005, on New Year's Day, Arashi released their first concert DVD since How's It Going? in 2003 titled 2004 Arashi! Iza, Now Tour!!. From July 26 to August 24, they embarked on a summer tour titled One to support their album of the same name. One of the highlights of the tour included the live performance their first single of the year, "Sakura Sake", on top of "Johnny's Moving Stage" (ziyanizumubingusutezi, Janizu Mubingu Suteji), a transparent stage that travels over the top of the audience. Invented by Matsumoto, "Johnny's Moving Stage" has since been used in a majority of Arashi's concerts to allow them to move closer to the audience seated at a distance from the main stage. In November, Arashi released a theme song for the manga-based drama Hana Yori Dango, which starred Matsumoto as one of the lead actors. "Wish" went on to sell an estimated 178,000 copies in its first week and over 300,000 copies by the end of its chart run, making it their first single to sell over 300,000 copies in total in nearly three years since "A Day in Our Life" in 2002.

Using a quote from the above article, answer the following question: What caused the decline in sales?
Starting in 2003, the group would only release two singles every year until 2007.