Simhika was protecting Lanka from the ocean. Lanka was inaccessible ,Simhika was also a factor to it. Simhika had a boon from Bramha that she could pull any creature by its shadow alone. she had the capacity to act on a shadow to paralyze the creature flying and swimming in the ocean. Since it was a boon from Bramha, even Sampati , angad, sugreeva LAkshman etc could not have crossed the ocean for a dialogue with Ravan.
Only Hanuman was capable. Hanuman too was captured by his shadow by Simhika ,but his capture was partly due to Hanumans' intention to honour Bramha's words .She was simhika for name sake only, Hanuman was Jeevasimha ,the devotee of Narsimha ,so it was no wonder he killed simhika to pieces.