Krishna himself along with Balarama, Satyaki, Pradyumna and whole of Yadavs combined couldn't do anything against Magadh's full might under Jarasandha..
He had unique boon from Rishi Chandakaushik to be invulnerable to weapons. 2 of his army generals named Hamsa and Dimvaka had same exact boon. In addition to this he gained many boons from Shiva which helped him conquer all countries in the contemporary world barring Hastinapura and Dwaraka. Hastinapura was spared due to Bhishma and Dwaraka was setup in Mountainous regions inaccessible to him.
Krishna to Yudhistira: -
It behoveth thee, O Bharata, to establish thy empire over all the Kshatriyas. But this is my judgment, O king, that thou wilt not be able to celebrate the Rajasuya sacrifice as long as the mighty Jarasandha liveth. By him have been immured in his hillfort numerous monarchs, like a lion that hath deposited the slain bodies of mighty elephants within a cave of the king of mountains.
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By defeating the kings with their troops and bringing all of them as captives into this city, he had swelled its crowds enormously. We also, O king, from fear of Jarasandha, at one time had to leave Mathura and fly to the city of Dwaravati.
If, O great king, thou desirest to perform this sacrifice, strive to release the kings confined by Jarasandha, as also to compass his death. O son of the Kuru race, otherwise this undertaking of thine can never be completed. O thou foremost of intelligent men if the Rajasuya is to be performed by thee, you must do this in this way and not otherwise.
Further he stated that Jarasandha is too tough to be beaten in a war, even by Dev's and Asuras (obviously a hyperbole). And therefore be subdued in a personal combat where his army or allies won't interfere.
He is incapable of being vanquished in battle even by all the celestials and the Asuras (fighting together). We think, however, that he should be vanquished in a personal struggle with bare arms. In me is policy, in Bhima is strength and in Arjuna is triumph; and therefore, as prelude to performing the Rajasuya, we will certainly achieve the destruction of the ruler of Magadha. When we three approach that monarch in secret, and he will, without doubt, be engaged in an encounter with one of us.
From fear of disgrace, from covetousness, and from pride of strength he will certainly summon Bhima to the encounter. Like death himself that slays a person however swollen with pride, the long-armed and mighty Bhimasena will effect the destruction of the king.
That could be mythological reasoning. Realistically, Jarasandha simply could've been an unparalleled military intellect under whose command Magadh empire was unbeateable.
Jarasandh by himself was merely immune to weapons but was vulnerable to defeat in physical combat. But under his leadership, Magadh was Strongest military force in that era.