There are many technical reasons. See this for why Roon does it support Upnp which is the largest protocol out there for music streaming.
https://community.roonlabs.com/t/whats-wrong-with-upnp/2101/3
There are many others out there and it gets difficult to support them all and they all have varying problems for what Roon wants to do hence they have developed and make available the RAAT protocol. See this for more on RAAT
https://community.roonlabs.com/t/whats-the-difference-raat-vs-airplay/6891/5