Many reasons. If you ditch bleed air from your engines, you need cabin air compressors; huge generators; increased electrical redundancy; big inverters + cooling; alternative engine starting; alternative wing anti-icing technology; bigger APU (potentially), and a few other things.
All these have a weight penalty.
Bleed air cabin pressurisation is a proven technology, although it has the risk that an engine issue in the compressor section can contaminate the cabin air supply.