Roku started as a streaming media box. You paid them money, they gave you a box that could play Netflix and Youtube. It was a simple transaction. Unfortunately, at some point they decided to start selling/giving their OS to TV manufacturers. This was actually nice at the start. You got a smart TV who's "Smarts" were designed by competent people. A revolution at the time. But the drive to drop prices lower and lower meant that there was no margin on the TV, which means Roku had to investigate other ways of making their revenue, AKA Ads and selling data.

Of course, the stand alone box probably would have went that way anyways, but at least with selling a dedicated box, there is a clear financial benefit without the need to get invasive.