Any IT transformation project like this is always hampered by people wanting customisations. Those customisations are usually to support Janet in office A who demands that she damn well isn't going to make changes to her workflow that she's done since she started in 1995.
Each airline on-boarded would demand mandatory features that they couldn't do without. Keep in mind these features weren't free, they were billed market-rate development charges - though the company didn't make money off that, the core business model was usage-based billing.
More than 80% of implemented features would go completely unused.