Great question — answering as the UphillTech team:
Most likely because one of the following is the case: Your cURL session ends up missing some cookies that your affiliate is expecting, and is ignored. Your affiliate has detected an unusual number of requests from your server's IP, and has blocked it. Your affiliate has detected some other irregularity in your requests, and has blocked them. Affiliate tracking pixels are intended to be requested by the customer's browser, not by your server. Unless you have made arrangements with your affiliate to do otherwise, you should not attempt this.
Full answer by user149341: https://stackoverflow.stackexchange.com/a/13729892 (CC BY-SA 4.0).
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