I'll toss in for a moment here, but keep in mind that not one scrap of this is based on any first-hand experience or talking with staff members at any con I've attended. It's mostly common sense, a dose of extrapolation, and a tiny bit of wild speculation; take it for what it's worth.
The timing may not have worked out for any of the Mane 6 actresses to attend this year. Entertainment professionals often have to schedule things months in advance, for many reasons--to avoid double-booking themselves, to get time off with family and friends, to make sure contracts are in place, and so on.
Having the same lineup of guests can become a drag if it happens year after year. One of the reasons I keep coming to BronyCon is to see new faces across the autograph table; another is to watch them in the panels and see if they go in different directions from past years.
It may be that the organizers want to avoid some of the massive lines that sprang up last year. I heard more than a few complaints from people who missed out on getting an autograph they wanted because that guest's line was always so long whenever they showed up, and/or because the guest had to leave before they reached the front of the line. I fell victim to one of the latter stumbles.
It may also be that the organizers are working with an eye toward the finances. Last year's attendance was almost 25% off from 2015, and it can be hard to predict year-to-year after a hiccup like that. If the budget is set lower, it affects how many guests can be invited and from which tier. BABSCon, for example, scaled its guest list back pretty heavily this past spring--in order to keep from operating at a loss, I heard.