Because Turkmen: They were being targeted and are inside Syria and related to Turks and want Assad out. Only Kurds and ISIS are allowed targets from Turkey's viewpoint. Putin wants Assad in, making all three good targets. Obama, weakly, wants Assad out and ISIS degraded while weakly supporting Kurds. Shooting the plane while attacking Turkmen is Turkey's signal for Russia to back off. Did you watch the video of Turkmen firing at the parachuting pilots? Did you know Russia mounted a Seals type raid and recovered the surviving pilot? The jet was either in Turkey for up to seventeen seconds or a five mile exclusion zone inside Syria where Turkmen live and that Turkey wants to defend but no one else recognizes. Protocol is to push back angry, not hit the kill button.
by the US (which dropped leaflets telling the drivers to run away) and one, less fussy, by Russia. The oil money adds up over a hundred million dollars in a year. Who is buying this oil? How about Erdogan's son? Those tankers have been an easy target for ages. (If you check the Russian story, the numbers are b.s. but the raid was real.)
How can anyone make sense of this when the facts above were heavily processed before release? Far more facts were overlooked. Instability seems the best prediction, rather than confidence that any one outcome will prevail. The bedrock motives of man include defending ties of blood and greed. I highlighted Turkmen (blood) and the son's oil (greed).
UPDATE: That the Turks shot down the jet and did so within 17 seconds – with the president, Recep Tayyip Erdoğan, saying he gave the order to fire himself – suggests very strongly they were waiting for a Russian plane to come into or close enough to Turkish airspace with the aim of delivering a rather pyrotechnic message.