Someone asked them why they dropped SMS a few years back and got a detailed reply: https://news.ycombinator.com/item?id=33258684
The most compelling bit here for me is RCS. Lots of devices will auto upgrade an SMS chat to RCS if they think that both ends support it.
Signal cannot support RCS (there's no API) so you'd end up with a situation where someone uses signal to send an SMS, the recipient device then replies over RCS and signal never gets the reply. Not great for usability.