Maybe because they identified how easy it would be for them to get nuked by Reddit?
A lingering question at that time was how imgur was paying their obviously enormous bandwidth bills, but /u/mrgrimm was always very vague about the source of his funding.
Long story short, it turns out that reddit had made an investment in imgur at one point while earlier funding was never made public. So reddit was banking on imgur being successful but never made that relationship explicitly public.
There were other weird relationship oddities too, e.g. imgur used to not allow pornography to be uploaded and linked to, except when the referrer came from reddit.com. (if you look up info on imgur and referrer tag weirdness, it's a common pattern)
After imgur started jumping the shark and reddit's entire executive team turned over, they decided to do their own image hosting service. But now they had a well funded plan-b to fall back on if their own hosting service didn't work out.
Imgur has always been very vague and shady about funding, revenue, growth plans. It's assumed that they just wanted to be a non-suck