Would it be the CPS who put a stop to it?
After the first court appearance, the magistrates would have adjourned to enable CPS to review the evidence and either (a) take over the prosecution or (b) take over and discontinue.
The court might not have been aware that CPS had already reviewed the evidence. For all it knew, the private prosecutor might have found some new, further evidence that hadn't been considered by CPS before.
With the huge caseload in that particular Magistrates court, I'm not suprised to hear that CPS didn't look at it until the day before the hearing. Seeing that it was the same old same old, with nothing new to meet the charging standard, they sent a last minute notice of discontinuance by email, I suppose.