I have a long experience with SQL, but recently began working with datawarehouse and OLAP technologies: building fact and dimension tables, that then are queried using MDX (MultiDimensional eXpressions).
The problem is that MDX works with a completely different logic compared to SQL, and it's a whole new learning curve even for someone with a strong SQL background.
Yes, MDX allows you to do things that would be hard or almost impossible with plain SQL. But sometimes it's frustrating to be hours around an MDX to do something you know you could achieve in minutes using SQL (ok, you can tell me to RTFM ...).
But why go on to the trouble of creating a new completely different language when you could build on SQL, extend it to add the features needed by OLAP applications?