Why did the Wadai Sultanate last so long?

Aug 2018
The Wadai Sultanate was a 16th century muslim state located in central Chad, west of Bornu in Nigeria and east of Darfur and Sennar in what is now Sudan. Wadai expanded it's trade connections in 1801 during the reign of Muhammad Sabun and increased it's military power though diplomatic relations with the Ottoman empire, importing chainmail armour, firearms and military advisors.

Sabun's successors, however, appear to have gradually neglected these relations with the 1909 Ouaddai war against French colonists being fought largely with cavalry and medieval weaponry.

What sticks out to me as peculiar is the length of Wadai's continued independence, with it's capital, Abeche, only being captured in 1909 and the Sultanate dissolving into French Equatorial Africa in 1912.

At a time when most other African Kingdoms had already fallen to European powers in the 19th century, what kept Wadai going against the French for more than a decade? The beginning of WW1 wasn't far off from their demise. Unfortunately further reading about this conflict is extremely limited.