Semi finals are usually among the worst attended games of the season - in Perth at least - so financially there's not great incentive to extend the semis. It's not that fans don't care, but it's easy to plan to attend a regular season game that has been on the schedule for ages, and there's more incentive to attend for those who have already paid for tickets via their memberships. With semis you often get little notice of when the game will be on.
TV-wise, most of the league's fans start to lose interest once their teams are eliminated from the comp, so I'm guessing that has a pretty significant impact on ratings.
If those sets of numbers don't justify it then I'm fine with keeping it best of 3. If one semi is a sweep and the other is a close series then the more games you have, the longer one team is sitting & waiting while the other is still battling for a spot in the GF, which isn't ideal either.