I've worked in the rail industry, so here's an explanation which might apply. (Since I don't know the region or the TOC, I obviously can't be sure):
When there are high wind speeds overhead cables swing about a lot. That increases the chances of the equipment on top of electric trains catching hold of the cables and pulling them down. (When that happens there's typically a mile and a half of cable which needs to be replaced, causing massive problems for everyone).
In order to reduce the risk of the cables being brought down, Network Rail reduces the maximum speed that trains are permitted to operate at along the relevant lines. (Trains normally running at 125mph or 100mph are initially limited to 90mph but that speed can be further reduced if the wind speed increases). That, in turn, reduces the number of 'slots' available for trains to operate on those lines.
TOCs with high-frequency services on those lines are then forced to cancel services (because they're simply aren't enough slots for them to run in), while TOCs with infrequent [or diesel-operated] services on the same lines aren't as badly affected.