One reason is that the Spurs themselves don't want it, another is that their style from 1999-2008 made it difficult for most to appreciate.

The Spurs PR team is also famously closed off and doesn't give media much access (likely at the request of Pop, Duncan, and co.). It's hard to give a team attention if they don't want it.

I love the Spurs, but I can totally understand why non-Spurs fans would think the 1999-2008 teams played a boring style, other than in the Suns and Mavs series. We played at a slow pace and grinded teams to victory. It also didn't help that our NBA Finals matchups were either against teams totally outclassed (Knicks, Cavs), or defensive-minded teams (Pistons, Nets).

If the Spurs had always played the beautiful game style of the 2012-14 teams then I think they would have had a lot more fans, like the Nash Seven Seconds or Less teams. Even Jokic's Nuggets, who are in a small market and have star player that doesn't do much media, gets a lot of love because they're entertaining to watch.

But because of the first decade of the Duncan Spurs, plus Duncan's disinterest in showing his personality in public (I love Duncan but he rarely gives soundbites like Embiid or Draymond), the idea of the Spurs being "boring" kind of stuck. Hence the lack of media attention.