Just wondered why this is
Not all, but some private tracker operators are just children that have a little Napoleon complex and like to dictate that users strictly conform to their preferences for torrent clients. Usually they will have various nonsense reasons that make little sense, but what it really comes down to is they want to be "big boss" somewhere, and this is one of many ways they try to do it.
A few of the really whacko ones even like to complain that Tixati gives users the ability to spoof peer-id and tracker headers, which effectively gets around such stupid client blocks, and therefore it should be banned because it interferes with their power to be a little dictator. Some of them even had the nerve to come into this very forum and DEMAND the developers remove the feature, OR ELSE be banned from their two-bit private tracker nobody give a crap about, which already bans all non-utorrent clients anyway. Obviously the devs don't even pay attention to these pests, thank goodness.
BTW it is easy to get around these blocks but I forget how. It has been so long since I needed to bother. If you search the forum for "peer-id" or something like that you will find out how to do it.