Delta has an app specifically designed for producing a machine readable boarding code, Spirit does not. Delta has programmed most of its scanners to read that app code, but not all. There are still airports where a paper boarding pass is issued by Delta.
Spirit obviously hasn't upgraded its systems to use machine readable app codes. And so will continue to use paper ones until they do that. They may well be working on it, but it takes time & money.
As for scanning the pdf barcode off your phone, that is determined by the abilities of scanning machine at the airport gate. Not all scanners can successfully read barcodes off phone displays. The same issue occurs using electronic coupons at grocery stores, sometimes the clerk has to manually enter the code number because the laser can't read the phone display. So can you imagine how long it would take to board an aircraft if the gate agents had to manually enter everyone's barcode from their phone ... hence paper boarding passes speed things up.