Anyway, it is worthwhile to practice writing recursive solutions (beginning with Scheme) to understand possible pitfalls in case of non-tail recursion, and then, proudly, switch to fixed-space looping techniques.)
* some platforms don't provide useful support (in the History it were some stack-oriented Lisp Machines, nowadays it is the JVM - see ABCL)
* there is no consensus how it should be invoked, how its effect should be reported by a compiler, how the user should be noted about limitations (example: LispWorks limits the number of variables), ...
* it is not a good fit with Common Lisp, since the language supports all kinds of dynamic constructs: dynamic binding, unwind-protect, ... It is not clear how this should interoperate with TCO in CL.
* it makes debugging more diffcult (backtraces, trace, ...)
Pascal Costanza has an interesting opinion[1] on why TCO wasn't included in the ANSI standard.