That's actually not true, Obama was called for impeachment many times. However their arguments held little water.
You're not telling me that, if presented with evidence like the Trump Jr mails, the anti-Obama crowd wouldn't have gone berserk ? Fox News would be, like CNN currently, hammering the "impeachment" narrative 24/7. Both sides, the anti-Trumps & the anti-Obama, are equally enraged by the opposition president.
The fact is, with Trump there is way more evidence of ethically questionable activities than with Obama. Trump is a low hanging fruit, he can't hold his tongue & his staff is generally incompetent, making him an easy target for hit pieces & calls for impeachment. Obama was more discreet & resourceful. Which is, arguably, worse.