Slave-sourcing for the army collapsed due to appalling mortality rates.

Evidence

It had earlier been realized that the army sent to the Sudan was much larger than the number of slaves captured, which defeated the purpose for which it was sent in the first place, and so it was decided that for every 1,000 men sent 3,000 slaves must be collected. However, even this principle now did not make sense, since those who had been collected were in very poor health and were dying “like sheep with the rot.” In desperation Mehmed Ali wrote to Boghus Bey, his Armenian advisor on foreign affairs, ordering him to hire a number of American doctors to treat the slaves. These were preferred to European physicians since they had experience of dealing with “this race” (bu tayfa). — Khaled Fahmy, All the Pasha_s Men_ Mehmed Ali, his…, 443