Leeds are unique, but the other teams have all had promotions or played in the premier league relatively recently.
Their fans have had some good highs, to counteract the lows, which keeps the hopes and interest levels going. We had 35 years of mediocrity, apart from 87, followed by relegation and 11 years more rubbish. In all that time, there has been very little to capture the interest of new supporters, whilst the existing fan base had diminished.
Any success is welcome, it offers an opportunity to capture new young fans, before they attach themselves to bigger clubs. A fan base is not created overnight, it builds, or decays, over decades.