First let me address those who are asking for Type Classes and Dependent Types. These would make the compiler too slow. The F# compiler works in the 100s of ms and the 100s of MBs ram. Where Scala is in the 1000s of seconds and the GBs of ram. People will give up on code completion if it takes more than 500ms and they generally don't like waiting longer than 200ms. Separately Martin Odersky (the creator of Scala) laments that the more advanced language features attracted the wrong type of people to the language and flooded the internet with overly academic tutorials.
I put a lot of the blame for the lack of F# adoption on Microsoft. Part of this is to do with the Dev Div vs Windows war that almost deprecated the whole .Net ecosystem. If Windows 8 wasn't such a disaster it's likely .Net would have been deprecated and would never had been open sourced. (at least something good came from Windows 8). Part of this war also ended with a lot of the best .Net people leaving to go to Facebook. This brain drain has really hurt Microsoft and I doubt they'll ever recover. Most of the old hands that know better are aging out and the new people at Microsoft don't know any better.
As for tooling, Microsoft has done and continues to do a bad job with F# tooling. There is is a not invented here culture at Microsoft Redmond which includes being hostile to Microsoft Research. Visual Studio is a bucket of bandaids and it takes an army of people just to keep it going. F# is given a low priority. It's worse with the latest release because they've gone overboard with compiler features with the Rosario release and have made it dog slow and painful to use. I hope they can fix it with updates but who knows. You can still get older versions. It's very possible to build a really good IDE around the F# compiler but unfortunately Microsoft refuses to do so. Thankfully, it looks like there are some good alternatives becoming available.
In comparing Scala and F# adoption, Scala has Hadoop/Spark which is a killer product that can earn a consultant a lot of money. The fact that C# is a better language that Java means there are more Java refugees going to Scala than there are C# refugees going to F#.
Thankfully Don Syme has made F# completely open source. Now with .Net being open source we no longer have to worry about Microsoft shooting itself in the foot again.
I make my money writing a product written in F# and can attest that F# is one of my big competitive advantages. I highly recommend it.