Why are TGeneric<Base> and TGeneric<Descendant> incompatible types?

I have started using of generics in Delphi 2010 but I have a problem when compiling this piece of code:

TThreadBase = class( TThread )
...
end;

TThreadBaseList<T: TThreadBase> = class( TObjectList<T> )
...
end;

TDataProviderThread = class( TThreadBase )
...
end;

TDataCore = class( TInterfacedObject, IDataCore )
private
  FProviders: TThreadBaseList<TDataProviderThread>;
...
end;

Then I have some nested procedure:

procedure MakeAllThreadsActive(aThreads: TThreadBaseList<TThreadBase>);
begin
...
end;

And finally I want to call this nested procedure in the code of TDataCore class:

MakeAllThreadsActive(FProviders);

But compiler does not want to compile it and it says ('<>' brackets are replaced by '()'):

[DCC Error] LSCore.pas(494): E2010 Incompatible types: 'TThreadBaseList(TThreadBase)' and 'TThreadBaseList(TDataProviderThread)'

I do not understand it although TDataProviderThread is descendant of TThreadBase.

I had to fix it by hard typecasting:

MakeAllThreadsActive(TThreadBaseList<TThreadBase>(FProviders));

Does anybody know why the compiler says this error?