The reason is that there are no abominations for Wave 8 Collectors Gold. They bleed the wave budget very rapidly when they are in a wave. They are something like 40 points a piece, when a Scion is only 75. A Praetorian is worth 120, so killing 3 Aboms is the same as 1 Praetorian. Troopers and Captains are only worth 25 and 35 respectively.
That's part of it, yeah, but for me it's more of a mooks vs bosses question. Unlike many, I don't mind Collector bosses because they're easy to combat and they're easy to avoid once you've learned their tricks. Collector mooks, on the other hand, have so much firepower that they rival the BS level of the Geth.
Wave 7 has one active Captain and the rest are Scions and Aboms while Wave 8 has 2 of each boss (IIRC) and the rest are mooks. So from my perspective, wave 7 is easier.