He is expensive, constrains your mana, and dies to bolt. The format has been slowly speeding up, which has limited his usefulness. Without Birthing Pod to cheat him out, he just isn't as good, and Splinter Twin as gotten away from the all in combo list, opting to maximize of value.
Some Twin lists still play a copy or two, but as the deck evolved from an all-in combo deck into a control deck with a combo in it, the need for redundant combo pieces was lessened. They usually don't play the full eight 2U guys anymore either. As far as Pod goes, the Melira version of the deck was ultimately judged better and the Kiki deck faded. And then, of course, Pod got banned. Collected Company is the "replacement", but you need your combo pieces to cost three or less for that. RWU Kiki Control was a thing for a while, but right now RWU of any flavour is not great.
So basically he just doesn't have a real home right now. At some point he may see some play again, but he costs five mana and dies to Bolt. I wouldn't expect him to take over the format.
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Decks I'm playing in Modern right now:
URB Grixis Reveler (http://www.mtgvault.com/supast4r7/decks/modern-grixis-reveler/)
UB Faeries (http://www.mtgvault.com/supast4r7/decks/ub-fae-2/)
UW Azorious Control (http://www.mtgvault.com/supast4r7/decks/modern-ojutai-control-2/)
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So basically he just doesn't have a real home right now. At some point he may see some play again, but he costs five mana and dies to Bolt. I wouldn't expect him to take over the format.
http://www.themeadery.org/articles/crash-test-modern-daily-with-kiki-chord-ep-14/