Up until some 5 years back, UX really was afterthought to them. Why? Answer is in how enterprise software was (and largely still) sold. Buying was done by CIO, based on all parameters (account relationship, FOMO, $$$ discounts, functional reliability etc.) except user acceptance.
SAP simply didn't have incentive and KPI to see UX as focus. Users were "trained" to use SAP after rollouts.
With what is fancy termed as "consumerization of IT", SAP has been forced to change. They've been trying their best with Fiori v1, v2, v3 since then. Change takes time to get hold in SAP world, specially given large on-premise installed base, where customers take years to adopt a release upgrade.