In instances like these, those in power go after the low hanging fruit, the quick win. In the case of the doctor, they had a case - what was described in court could have been judged illegal as the FGM law does prohibit reinfibulation, and even the one stitch could count even if the motive wasn’t for FGM purposes, as long as it was not a ‘necessary surgical operation’ - but more importantly they had evidence. The single stitch was done by an NHS doctor, in one of the country's most famous hospitals, and the doctor asked a senior colleague if he'd followed the correct procedure, having been trained to deliver babies, but not on what to do if the mother has complications due to childhood FGM. Records were kept, and witnesses were reliable: little investigation needed to be done.
The FGM incidents that society wants to prosecute, however, aren't like this. They're done by backstreet surgeons, or arranged by families that take their child elsewhere to get it done. Presumably these incidents are not logged by any bureaucracy but are kept secret. The only witnesses do not want to talk. They might be in favour, in fear, or just want to keep the family together. The crimes may come to light years after they happened, so getting proof of who said and did what is much more difficult. It's no wonder that prosecutions are difficult.
A genuine anti-FGM action requires a lot more than legislation. It's cheap to pass laws, and expensive to actually enforce them. Like The Wire's major investigations, a good prosecution would need undercover work, maybe some communication interception, and certainly some people willing to talk from the inside. Resources, mainly 'man-hours' have to be allocated: the work has to be prioritised. Given that current estimates are that the police budget has been cut by 20% over the last 5 years, something would have to give. Who'd like to make those choices? Going after FGM crimes? Rape? Burglary? Terrorism? Fraud?