We did get 172 investigations into Hillary over far less damning things. Probably about time congress opens up a full, high profile investigation into Trump's ties to Russia.
If the Trump cultists honestly believe there is nothing there, they shouldn't be worried about an investigation. In fact, they should want it, as an investigation proving his innocence (which Trumpkin cultists so firmly believe) in the matter would be a great boon to the credibility of the Trump administration. However, their vehement protestation of an investigation seems to indicate that they're not so confident Trump is innocent in the matter.
In fact, many probably think Trump being compromised by Russia is a good thing, and while knowing that it's highly illegal, don't want Trump removed from office for it, and would rather just have their traitorous leader stay in power.