Other than the obvious reason, I mean:
Source to @jaketapper: 2 reasons Comey was fired
1. Comey never provided Trump w assurance of loyalty
2. FBI's Russia probe was accelerating pic.twitter.com/vG5bmTCPeg— The Lead CNN (@TheLeadCNN) May 10, 2017
"It was done to send a message … It's like the horse head in the bed" – Fmr sr intel official tells @KenDilanianNBC re Comey firing. pic.twitter.com/3OLH2MOkWL
— Katy Tur (@KatyTurNBC) May 10, 2017

In 2014 Jared Kushner told us that much of the financing for Trump’s golf courses came from Russian oligarchs.
It’s common knowledge that Russian oligarchs routinely launder their money through greedy businessmen like Donald Trump.
If those oligarchs decided to stop financing Trump or call in their loans then the Trump “empire” would collapse.