Four main reasons I would think
1. Manchester is in varying degrees of lockdown week to week so planning an event there at the moment is very risky and has no guarantee it will happen.
2. Ipswich has around two thirds more capacity than the NSS so when percentage restrictions are applied you can still get a reasonable (and more importantly viable), crowd in..
3. With Kings Lynn, Peterborough and Mildenhall all 'local' you have four lots of Speedway starved fanbases to pull from, which (with a good few from around other parts attending) hopefully will ensure all tickets available will be sold..
And 4. Ipswich might be 'in the middle of nowhere' but does traditionally at Easter get one of the largest crowds (if not THE largest single crowd), of the season at any track, so obviously have a strong underlying fanbase who show they will attend on one off occasions..
Put it all together and I would think the size of the venue, the amount of awareness of Speedway that exists in that geographical area, and the fairly low impact of Covid 19 to that part of the country, ticks a lot of boxes when it comes to planning and putting on an event that needs a crowd to be a financially viable...