[neurotech1]

Its about budgets, both on development and on procurement and operational costs. The Su-27 series and the F-15 are relatively expensive jets to procure and operate, and the F-16C (or F-16A MLU) are a good middle ground budget wise.

The F-5E/F Tiger [0] was one of the most under appreciated fighters on the market in the 70s and early 80s. The was limited in A/A victories in real combat because of lack of opportunities, although successfully used at TOPGUN and FWS at a low operating cost. Although marketed as a LWF, The F-5 was actually a multi-role aircraft, and used in combat as a strike aircraft.

IMO the F/A-50 [1] is the most under-appreciated advanced 4th gen fighter on market. At around $30m its half the cost of a new F-16, although a refurbished F-16 is viable for budget constrained Air Forces.

[0] https://en.wikipedia.org/wiki/Northrop_F-5

[1] https://en.wikipedia.org/wiki/KAI_T-50_Golden_Eagle