If that were the case, input methods by Chinese companies like Sogou wouldn't support Cantonese either. The opposite is true: here's a description of how to enable Cantonese Jyutping input in Sogou for iOS: https://jingyan.baidu.com/article/60ccbceb100b7c25cab197c4.h...

I think the more likely explanation is that Apple has a small team supporting all of the world's languages, mostly focusing on widely-spoken ones, while Sogou has a small team focusing on languages widely spoken in China, so their Cantonese support is better.