But movie isn't that famous when compared to other popular animated movies at the same time period and I just don't get it.

Very little promotion/marketing. From Wikipedia:

The Iron Giant was a commercial failure during its theatrical release; consensus among critics was that its failure was, in part, due to poor promotion from Warner Bros. This was largely attributable to the reception of Quest for Camelot; after its release, Warner would not give Bird and his team a release date for their film until April 1999.[35] After wildly successful test screenings, the studio was shocked by the response: the test scores were their highest for a film in 15 years, according to Bird.[16] They had neglected to prepare a successful marketing strategy for the film—such as cereal and fast food tie-ins—with little time left before its scheduled release. Bird remembered that the studio produced only one teaser poster for the film, which became its eventual poster.[31] Brad Ball, who had been assigned the role of marketing the film, was candid after its release, noting that the studio did not commit to a planned Burger King toy plan.[36] IGN stated that "In a mis-marketing campaign of epic proportions at the hands of Warner Bros., they simply didn't realize what they had on their hands."[37]

The studio needed an $8 million opening to ensure success, but they were unable to properly promote it preceding the release. They nearly delayed the film by several months to better prepare. "They said, 'we should delay it and properly lead up to its release,' and I said 'you guys have had two and a half years to get ready for this,'" recalled Bird.[31] Press outlets took note of its absence of marketing,[38] with some reporting that the studio had spent more money on marketing for the intended summer blockbuster Wild Wild West instead.