If Sri Hanuman can carry Sanjeevini mountain why didn't he just take a similar mountain carrying all Rama's army to Lanka?

Yes. Shri Hanuman can do such a feat.  However, that is a last resort.  He did not do it for a reason.

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Seeta understood the strength of Shri Hanuman. But, Seeta did not want Sri Hanuman to take her to her husband. She wanted her husband Shri Rama to fight the battle, win her in the battle and get fame for that act.

Seeta asked Shri Hanuman to see that Shri Rama comes to Lanka, win the battle and take her back to Ayodhya.

कामम् अस्य त्वम् एव एकः कार्यस्य परिसाधने |
पर्याप्तः पर वीरघ्न यशस्यः ते बल उदयः || ५-३९-२८

"O Hanuma the annihilator of hostile enemies! You, as one alone, is capable of fulfilling this task. Your act of fulfilling the result is well-known."

बलैः समग्रैः यदि माम् रावणम् जित्य सम्युगे |
विजयी स्व पुरम् यायात् तत् तु मे स्यात् यशः करम् || ५-३९-२९

"If Rama defeats Ravana with his entire army in battle and as a champion takes me back to his own city, it will be quite worthy of him."

तत् यथा तस्य विक्रान्तम् अनुरूपम् महात्मनः |
भवेत् आहव शूरस्य तथा त्वम् उपपादय || ५-३९-३१

"For that reason, you act in such a way, that Rama the high-souled man, valiant in battle, may exhibit the prowess befitting of him."

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That was why, though capable of taking the entire vanara army to Lanka, without constructing a bridge, winning the battle single-handedly, Shri Hanuman remained as a spectator. He advised and assisted Shri Rama and Lakshmana, when asked for.

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It was their battle and they have to toil and win the battle.

Thus, Shri Hanuman assisted Shri Rama in constructing the bridge, crossing the bridge, fighting the war and finally winning the war.

He honoured the request of the Sita.