Because IMHO it's the de-facto standard UUCP implementation at this time. There are thousands of sites with experienced admins and there are many places you can get incredibly good information concerning the HDB setup. The uucp-1.04 that's in SLS 1.02 and later has all three modes of config files built in. The following workaround is ugly, but it does work, if you want to run Taylor configs from binaries that don't have it built in. If you get chat script timeouts, typically connecting to other Taylor sites, a pause after login can fix this.
Some kernels experience modems hanging up after a couple of seconds.
The following patch sent by Ian Taylor might help. If you want to do anon uucp in HDB mode, you'll have to recompile the sources with just HDB defined. Also, Taylor in HDB mode seems to be sensitive to white space and blank lines. To be safe, make sure that there are no blank lines or trailing spaces in the Permissions file. That tells which dialer to use in Dialers.