Is that because opposing supporters will fight if they're in the same town together or so one city's force doesn't need double the number of officers on duty on the same day? They don't cancel the FA Cup if it's Liverpool vs Everton.
An example. If Arsenal play Spurs at home, then there are 57,000 Arsenal fans and 3,000 Spurs fans in the premier league. In theory, in the FA Cup, Spurs would receive an allocation of 9,000, but there is no way the police will allow that, so the allocation would be cut (a security decision and Arsenal would be fined). The kick-off time of the match is subject to police review - they have a strong preference for it to not be a late kick-off to prevent pre-match drinking. Their approach on this has softened lately though.
The Spurs fans arrive by a dedicated entrance with extra police and horses. They are often prevented from leaving at the end of the match to allow crowds to disperse first and then they are given an escort out of the area.
In London, there is the policing available to police more than one event (maybe not in Liverpool), so it would be possible for both Spurs and Arsenal to play at home, but then there would be 57,000 Arsenal fans and a similar number of Spurs ones all out in North London on overlapping transport routes. This could, of course, be policed, but it would mean that they would need more police in total than they would if the matches were on different days, so they play on different days.
All of these precautions are taken for Arsenal and Spurs. They're rivals, but pretty docile in the grand scheme of things and there's rarely any trouble.
I'm cynical of many things the UK police do, but they are pretty good with football games.
When Arsenal play away at more hostile places such as Roma, they arrange special buses to transport fans directly to the stadium. The buses are often attacked. It's generally a disgrace. Nothing seems to be done about it. It's not headline news.
Presumably buses were an option for the Villa match; however, in addition to all the Villa fans, the police would have had to deal with protestors who believe that Israeli teams should not be participating and the thoroughly pleasant Maccabi fans. It's worth noting that the CEO of Maccabi has admitted they have a racism problem. I believe the police said, the match could go ahead if they had more police available. This all seems pretty standard stuff to me and not worthy of government comment.