In the 2019-2020 House Impeachment inquiry into President Donald Trump, many requested witnesses did not testify nor provide documents. Many of these witnesses defied subpoenas either voluntarily or due to the White House's stance toward the inquiry.
In addition, there were also over eight witnesses that were proposed by Rep. Devin Nunes but were ultimately turned down by Rep. Adam Schiff. Similarly, there were also 8 witnesses called by Rep. Doug Collins that were also subsequently turned down by Rep. Jarrold Nadler.
The turning down of the witnesses in both cases may make sense, or it may not make sense from the House's point of view. However, the simple act of turning down witnesses, however justified it may have been, makes the argument of "the House stopped certain witnesses from testifying" a truthful statement. This statement alone would lead a third-party observer to possibly cast doubt on how thorough the information of the House Inquiry really was.
Regardless of the sensibility or frivolity, why the heads of both the intelligence and judiciary committees turn down witnesses? Please only give stated reasons.