Because 60-70% Ukrainians sometimes turn into phenomenally stupid people, believing cheap populist promises. Zelensky is our downgraded copy of Donald Trump. And despite all of this he still has 40% approval(We are multiparty country, so that's a big number). My hope is that by next spring he collapses entirely. Thankfully, he already lost half of his support.
While it does appear a lot of the population is lukewarm about Zelensky (37% rate his performance as merely "satisfactory"), he is also considered trustworthy by a 20 point margin. Additionally, being close to 40% in the first round is a very good position for him to be in considering that he likely would consolidate Timoshenko's support and the support of others in a runoff with Poroshenko.