While Namecheap hasn't provided the official statement, or even a hint to the reason why new registrations for .xyz domains are suspended, but that is quite the interesting news.
This paragraph is a good argument as to why, although the author acknowledges this is just pure speculation at this point.
We can only speculate but I imagine there is quite a lot of spam being distributed on .XYZ domains due to the low registration cost. It makes it attractive extension for spammers in addition to other lost cost new domain extensions. I’ve read and hear from IT professionals they have completely blocked many ngTLDs in company email filters. This is due to the fact they get so much spam email from new domains. Network administrators then set rules to block email from the entire extension. The sheer volume of complaints could be too much for Namecheap to handle. Again this is just speculation.
There is also an opinion that we will never know the reason.
Has .xyz been under a lot of bad press? You bet. Their 1-cent and 2-cent registration promotions have resulted in tons of people registering thousands of domains and a healthy portion of them were purchased for spam purposes.
On the other hand, you have companies like Google itself (abc.xyz), among others (xyz.xyz, ceo.xyz, hooli.xyz etc) who turned to .xyz and who rank in Google just as well. Perhaps these are just examples of simple cleverness and .xyz domain, but that doesn't mean a range of domains cannot be taken seriously as well.