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

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

While seeing off Sri Hanuma, Sita says as follows:

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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Sita asked Sri Hanuma to see that Sri Rama himself gets the glory for eliminating Ravana alongwith his followers and that includes construction of a bridge also.

So Sri Hanuma waited for Sri Rama's action.