The main reason is the clear split of responsibility between the railway undertaking and the infrastructure manager, which has been enforced by the EU Directives. The Control Command and Signalling (CCS) system has naturally stuck to the same logic, resulting into two separate ETCS trackside and on-board subsystems.
However this has to be put in perspective with the very high degree of integration of the legacy CCS systems, which have been developed on a national basis and with a speed/distance control philosophy closely linked to the underlying signalling system. For instance, the required safety level of the railway operation with legacy systems is obtained according to assumptions with regards to the braking (safety margins) that can vary drastically from one country to another one, that very often cannot be apportioned between trackside and onboard or even worse that are not clearly identifiable.
Therefore the move towards a unified speed and distance control, together with a clear split of responsibility between the infrastructure manager and the railway undertaking, implies that: