A little history on the exchanges. JINX has been around for much longer than NAP Africa. It was originally housed at a single location, IS Parklands and pioneered open, settlement-free peering in South Africa. In short, you had to pay to acquire data directly from other networks. However, settlement-free peering was not really a thing until a large player in the SA market, MWEB, decided to dictate the terms of engagement.
Back in 2010, transit was exorbitantly expensive and Rudi Jansen wagered that it would cost the likes of the big 5 (Telkom, IS, MTN, Vodacom and SAIX) significantly more to deliver content to his large user base via transit (at the time, the networks peered freely in London) than via peering in SA and that they would buckle to his demand for settlement-free peering. At the time, NAP Africa was in its infancy (as was Teraco), the ICASA Act had just allowed for ISPs to begin operating as Telcos and not every network had access to Parklands; however Jansen's bet paid off and if memory serves me right, 3 of the 5 gave in and peered immediately. This is one of the reasons uncapped packages began to appear. On a side note, the "Big 5" are still very difficult with peering and don't easily peer settlement free.
In general, exchanges are limited to a certain region and require ISPs to transit traffic between regions (this keeps them vendor neutral and costs low). Having multiple exchanges in a region is convenient as it allows for multiple fabrics for data to move on, creating redundancy and resiliency and in the case of JINX, allows peering between DCs for free.
Teraco's subsidised (free for ISPs) NAP Africa quickly gained momentum and the snowball effect soon took over. Cloud providers chose Teraco because it had more peers than the confines of IS Parklands (where JINX was still stuck). ISPs in turn wanted to move to Teraco because of the free peering and direct access to larger content providers and so on and so forth. Today NAP Africa is the 15th largest exchange in the world (by number of peers and I stand to be corrected), and Teraco is a Billion $$ company and is busy building their 3rd hyper-scale facility in JHB, so their free exchange bet paid off. JINX has expanded their network into most of the larger facilities in JHB (Hetzner Samrand, Africa Datacentres Midrand, Teraco Isando and IS Facilities) and continues to provide a large amount of free peering capacity to many ISPs.
Today, NAP Africa JHB Carries about 13-14 times more traffic than JINX:
NAP Africa Traffic:
https://www.napafrica.net/peering-traffic/ +- 700 Gbps peak
JINX Traffic:
https://portal.inx.net.za/statistics/ixp +- 50 Gbps peak
Port and Cross Connect Costs do make INX-ZA's (JINX, CINX, DINX) peering fabric slightly more costly for smaller ISPs, which is why some choose not to use them. NAP Africa remains free.