Because Slack is really expensive. Perhaps they concluded that an in house solution would be cheaper for them long term.

Slack is for "small" teams. They've got 10,000+ people in a 'group' since this isn't just for engineering staff, but also for drivers, customer support reps, etc.

These guys aren't split into lots of multiple teams under one big company, they're all just one 'team' with people leaving and joining fairly often due to driver turn-over.

Slack would cost ~$100,000 a month at those numbers as well (given enterprise level discounts).

Likely privacy, cost and performance.

For one thing Slack takes and exceedingly long time from app open to being able to see messages. The guys over at Slack spend more time accommodating for loading catch phrases like 'you look nice today' than actually loading faster..