The reason for ClickHouse is simple: it's fast. And we need to have it function like a TiKV coprocessor which support filtering and aggregation mainly and ClickHouse is good at aggregation and filtering. Also it might take more time and dirtier to do seamless / compatible integration to TiDB if Impala or presto on top of MPP layer. But the price we paid is implementing MPP layer by ourselves now.

Almost all the data lake based products loss full control over storage system. It makes them very hard to build delta-main engine we need. To make HTAP storage transparent to query layer, TiFlash need a lot more control over storage engine than data lake can provide.