I agree vienne, that’s essentially the reason: JWs believe Jesus is in a class of his own and therefore superior to the angels. He is an angel in the sense that he is a spirit creature, and “angel” is the term for a spirit creature, but he is far superior to all the other spirit creatures, both because he is the only-begotten, and because he was exalted to an even higher position after his resurrection. I think that’s the reason why JWs use term sparingly, because there is much more to be said about Jesus and more exalted descriptions that are appropriate than just the description “angel”.
On the other hand, the reason why opponents of JWs use the term “angel” so often to describe the JW view of Jesus is because it gives the impression that JWs in some sense belittle Jesus’ position, or view him as a “mere angel” - I’ve seen that phrase used a few times in anti-JW literature, but never in the WT. In one sense it’s fair enough, from a Trinitarian perspective, because no matter how exalted JWs view Jesus, they still think he is a creature, and for Trinitarians that just doesn’t cut it. Yet, I think it would be more honest for Trinitarians, who are describing the views of JWs, to acknowledge that JWs don’t view Jesus as a “mere angel”. They believe he is totally unique and far above the other angels. It’s fair enough for Trinitarians to dispute the fact that Jesus is a creature, but it would do them credit to also acknowledge just how unique and exalted JWs view Jesus.