Complex sentence: Due to lack of return on investments, it was not seen as viable to be kept operating and was the end result due to an entrance fee that had also been introduced which proved unpopular and resulted in a serious downturn in the number of visitors.
Split into simpler sentences: Due to lack of return on investments, it was not seen as viable to be kept operating. This end result was speculated to have been due at least in part to an introduced entrance fee which proved unpopular and resulted in a serious downturn in the number of visitors.