Because Oppo is a Chinese brand and until now has only really sold their handsets in Asian countries. The brand is quite big in China/Hong Kong though Apple, Motorola, LG, Samsung & HTC are BIG competitors there. Nokia & Blackberry are there too obviously but the whole wow factor behind them has obviously declined. There is so much choice out there.
Usually when people thing chinese made phone, they think of some cheapo bootleg Samsung or LG rip off. thats built using the same general design but made with massively inferior products to give you something that looks like a Note 3 or S3 Etc but at less than 50% if the price in most places. While these handsets DO work, they not for everyone given the fact that they are mainly made in unknown factories in china, some of these handsets are sold on amazon, ebay and can also be picked up on aliexpress & other similar dodgy sites like that.
Oppo is a lesser brand when it comes to LG, samsung or Motorola but the guy who designed the Find 7a also designed the HTC One M8 & OnePlus One. --- I could be wrong but thats what ive heard. the lead designer who originally came up with the HTC One M8 design left the company not long after and ended up working for Oppo, Im not sure where the OnePlus company comes into it but somehow they're all linked together.