I'd be curious the real reasons for this, because the reasons listed in this press release sound like marketing-speak.
This is undoubtedly at least in part due to a halo effect. If Alice and Bob have $0 Let's Encrypt certs for their blogs about, respectively, an obscure species of tree frog and restoring muscle cars, when Carol asks them if her new business "needs" an SSL certificate they are more likely to say "Yes" rather than "Not really" or "I don't know what that is". Even if most Carols use Let's Encrypt, any who don't are new customers for a commercial CA.