Mali's junta said on Friday it would resist any deployment of West African soldiers in the country and treat foreign forces sent there under a regional plan as "the enemy".
The ECOWAS force can't lead a counter-attack north and it is unneeded to protect the south from any attack from the north. Its only purpose would be to influence internal Mali politics. The junta would rather not have that, given it justified its actions to prosecute the fight against northerners. The junta has no way to make sure the ECOWAS force would want to reverse secession more than it wants to reverse a coup.
So Mali is too weak to fight the northerners.
ECOWAS is too weak and isn't welcome.
Only France could lead a credible force to spearhead a drive north.