Complex sentence: It was noted for decades as having the dominant share of the tampon market, challenged in the USA mostly by Playtex, J&J, Kimberly-Clark and briefly by P&G's failed product from the 1970s called "Rely".
Split into simpler sentences: It was noted for decades as having the dominant share of the tampon market, challenged in the USA mostly by Playtex, J&J and Kimberly-Clark. P&G previously also competed against Tampax with the failed product from the 1970s called "Rely".