Why have they been delaying Kizumonogatari for so long? Kizumonogatari is a movie based on a story of the same name in Monogatari Series light novel, which is part of First Season before Nisemonogatari. I don't even remember when it was supposed to air, probably two or three years ago. After the delay, they kept producing other stories in Monogatari Series like Monogatari Series: Second Season, Hanamonogatari and Tsukimonogatari.

Why did they put it off for so long? If it was because of production issue, like Hanamonogatari, why did they keep producing another Monogatari Series but not Kizumonogatari?
There's no doubt to the popularity of the Monogatari series that fans are clamoring for Kizu, but since it was delayed by production AND the immense popularity of Madoka Magika, what's probably happening now is that Kizumonogatari is likely being planned to be released towards the end of the franchises finale, as a reflective issue. 

No source for info, just an observation and estimation. Tsukimonogatari marks the beginning of the end of the series, the final arc so it should be within this year that we hear news about it.