I got to speak with some of the leads at Ollama and asked more or less this same question. The reason they abandoned llama.cpp is because it does not align with their goals.
llama.cpp is designed to rapidly adopt research-level optimisations and features, but the downside is that reported speeds change all the time (sometimes faster, sometimes slower) and things break really often. You can't hope to establish contracts with simultaneous releases if there is no guarantee the model will even function.
By reimplementing this layer, Ollama gets to enjoy a kind of LTS status that their partners rely on. It won't be as feature-rich, and definitely won't be as fast, but that's not their goal.
llama.cpp is designed to rapidly adopt research-level optimisations and features, but the downside is that reported speeds change all the time (sometimes faster, sometimes slower) and things break really often. You can't hope to establish contracts with simultaneous releases if there is no guarantee the model will even function.
By reimplementing this layer, Ollama gets to enjoy a kind of LTS status that their partners rely on. It won't be as feature-rich, and definitely won't be as fast, but that's not their goal.