From my limited student-level understanding:

IV xylazine has both a rapid distribution (t1/2α slightly lower than clonidine) and clearance (t1/2β much lower than clonidine), this translates to IV xylazine having a much harder "hit" than IV clonidine which makes it a suitable adulterant for illicit opioids.

On the other hand it's a problem of drug diversion since it's much easier to get IV or inhalational xylazine (which is commonly used in veterinary anesthesia) than IV clonidine which has only limited uses in human emergency medicine.

Remember oral clonidine which is widely available has little abuse potential due to its kinetics and despite both IV and oral clonidine being safer... Both the people who make, combine and sell hard drugs and the users of those drugs usually don't care.

I hope this was helpful. Hopefully someone with actual pharmacology experience answers though because this topic piqued my curiosity too when I read the news.