Wikipedia says that last Roku with an SD slot came out in 2017, so I assume this is an older model. Why did they do it then? Probably because memory was more expensive, and extensibility is often seen as a selling point.
It was a cost thing at the time, please people may not have had as many channels loaded, or switched between all that many at a time. I believe, other than the basic Express, all models now have 4Gb of channel storage.