I'm guessing that complexity comes into it. To make pedelec use legal requires adopting the 1983 EAPC regulation, as amended 2015, plus adopting the two and three wheel motor vehicle type approval legislation, since this contains the exemption for pedelecs not being considered motor vehicles.
Of course the latter includes a very large range of vehicles being authorised for type approval, including such as large Harley Davidson trikes for example. It also contains a number of exemptions which Stormont might not agree with.
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It's my understanding that the Northern Ireland legislation exempting pedelecs is already in place, in fact has been since 1995. The issue is that the legislation refers to "an electrically assisted pedal cycle of such a class as may be so prescribed", however the class was never defined.
This has always been claimed to be an "oversight", and would, I assume merely require a reference to the EU definition. I may be wrong, but I think the existing EU type approvals have also been adopted for 2 and 3 wheelers, so the EPAC definition could probably have been done at the same time.
The other point to note is that when exempting pedelecs from MOT requirements, they opted for the phrase "a vehicle which is incapable, by reason of its construction, of exceeding a speed of 15.5 miles per hour on the level under its own power" rather than actually defining a pedelec properly, which could probably then have been used for the 1995 legislation too.