It's the pure truth and nothing else. Heavy Cavalry was the most picked commander in the WCS 2019, now go and check how many Rangers were built. But wait, I'll spare you the trouble and just say it: 2. Out of 22 picks of the Heavy Cavalry doctrine, only 2 Rangers were built.
It's not so different in the ladder either. Heavy Cavalry is certainly meta and sees the most use but despite that fact, it's been a while since I saw the Rangers last time.
You think it's because rangers are bad? when they can 1v1 obersoldaten with thompsons? Or because USF is forced into multiple rifles, gets forced to use free officers and then doesn't want a 6th and 7th infantry squad?
Not trying to be rude, but there's more factors in play than just "rangers not strong enough therefore not picked"