I will agree that the percentage of votes cast for UKIP has declined in the two recent by-elections.Here's some numbers for you... 3 recent by elections totals.
Conservative (inc. Zac Goldsmith) - 53,521
Liberal Democrat - 35,727
Labour - 10,643
Minor parties/independents - 6,901
UKIP - 5,780
Of the votes cast above, these are the percentages/shares:
Conservative - 47.54%
Liberal Democrat - 31.74%
Labour - 9.46%
Minor parties/independents - 6.13%
UKIP - 5.14%
So UKIP don't appear to be too popular in all post referendum elections.
There are a multitude of reasons why that has happened and we can each have our own opinion on that. The reality is, no one knows what the actual answer is. Certainly not me or you. Fact.