The reason is simple poloniex is dead.You might wonder why and what is the base of this statement.
Read the reasons below and find out the truth yourself;
1.The security, they say they have a 2FA verification and that is true.But the kind of 2fa they are using is a piece of shit .
the reason is if suppose your computer is attacked by some malware like METASPLOIT for a remote session ,you could easily lose money cause they only require email confirmation for withdraw ,so if your email is opened in your computer (90% people does have opened email account in their personal computer) your money is gone for ever.
On the other hand Bitfinex have white list withdraw address and in cryptopia you can have a different email account for the withdraw confirmation email.
2.Their support ,My goodness this is the worst part one can ever have.
Just search (poloniex support not responding)
https://www.google.co.in/search?client=firefox-b&dcr=0&q=poloniex+support+not+responding&spell=1&sa=X&ved=0ahUKEwivyuuc4sPWAhUGo48KHePXD18QvwUIJCgA&biw=1366&bih=610
in google and see your self ,some tickets are pending for 6-8 months without any reply.
Suppose one day you have noticed some suspicious activity in your account and you instantly freezes your account.
By the rule of poloniex you have to now contact poloniex support and create a ticket to open your account again.
DAMMMMMMMM and you will be locked with all your money for 6-8 months without reply ,sounds horrible HUH!!!
What if someone really hacks your account or somehow your withdraw stuck in block chain ,There is no one to guide you.
3.Backdated website ,every other exchanges have evolved and have afresh website with lots of function.
but in poloniex you can often face that the website sucks and you have to refresh it to watch live chart .
4.All other websites have better charts for day to day trading ,(though we all use trading view) still the integrated chart in website helps a lot while trading.
5.Calculation of fees, This one is another big shit ,all other website always calculate the bid and ask price in the bid and ask window after the fess ,even you can check the fees yourself at the bid or ask window.This helps a lot for traders as you know when you made a profit and when you made a loss straight way.
But sadly in poloniex they donot have that you have to manually calculate the fees after every trade and sometime you may loose cause in day to day trading we may want to have only .10% profit in case of a reverse trend.
A fees deducted total amount helps a lot in this kind of trade .But in poloniex you will be totally blind in terms of fees while trading.
Hope this analysis helps you and you do your own research on this .
You money is only yours if you can store that in a safe place.
Always think before choosing a exchange as they have your private keys .
From my trading experience.
1.Bitfinex 2.Bittrex 3.Cryptopia
these are good exchanges 
LASTWORDS:Poloniex.com may be the oldest exchage but they lack in up gradation and security they never evolve to new technology .
