That's because F# 1.0 was bascially a more powerful language than C# 10.0 (C# still doesn't have proper pattern matching for example, or discriminated unions)
Named union type fields
Extensions to array slicing
Type inference enhancements
Printf on unitized values
Extension property initializers
Non-null provided types
Primary constructors as functions
Static parameters for provided methods
Printf interpolation
Extended #if grammar
Support for 'fixed'
Tailcall attribute
Multiple interface instantiations
Optional type args
Params dictionaries
Struct tuples which inter-operate with C# tuples
Struct annotations for Records
Struct annotations for Single-case Discriminated Unions
Underscores in numeric literals
Caller info argument attributes
Result type and some basic Result functions
Mutually referential types and modules within the same file
Implicit "Module" syntax on modules with shared name as type
Byref returns, supporting consuming C# ref-returning methods
Error message improvements
For features added see https://en.wikipedia.org/wiki/F_Sharp_(programming_language)
Named union type fields Extensions to array slicing Type inference enhancements Printf on unitized values Extension property initializers Non-null provided types Primary constructors as functions Static parameters for provided methods Printf interpolation Extended #if grammar Support for 'fixed' Tailcall attribute Multiple interface instantiations Optional type args Params dictionaries Struct tuples which inter-operate with C# tuples Struct annotations for Records Struct annotations for Single-case Discriminated Unions Underscores in numeric literals Caller info argument attributes Result type and some basic Result functions Mutually referential types and modules within the same file Implicit "Module" syntax on modules with shared name as type Byref returns, supporting consuming C# ref-returning methods Error message improvements