TCAS or not is based on need and risk. For example the 8.33 kHz spacing radios, IIRC (t may have been something else) were restricted to aircraft using upper airspace, this included the F3 Tornado but excluded the GR1 Tornado.
Why? Simply a case of spending money only where absolutely necessary and applying States exemption to occasional upper airspace users.
TCAS might be 'inexpensive' but by the time it has been trialled to death at Boscombe and then retrofitted to several hundred aircraft - strange or odd, but true - the cost becomes extremely significant.