My perspective as someone who buys seats on the resale market, it's a combination of a lot of things (price, playoff fatigue, and waiting for the Finals), but the primary one for me is a lack of a marquee opponent.
I think demand would be different if the C's faced a healthy Jimmy Butler in Round 1, followed by healthy 60-win Sixers or Bucks.
But the C's playoff opponents so far have felt like a Wednesday night game in January against a lottery team. Not to disrespect the C's opponents, but the expected good game/series against a formidable opponent who is a threat to beat the team in 7 just isn't there (at least not yet, and hopefully never).