I think Ashcroft does have more pull than people realise.
Urban Hymns was massive at the time, so theres always gonna be nostaligia for that reason. Far bigger than any Stone Roses album really, commercially at least. Also I think a lot of it has been to do with the fact he hasn't always been active during his solo career. He has been in recent years but at one point he went 10 years without doing any solo album or many gigs. His albums generally chart in the top 2 or 3 too, even now, so he definitely has a captured audience.
Plus, it's not like he's doing the usual circuit every year, so I reckon people probably come out to see him for that reason, because they can't catch him that often.
Can't say his solo stuff does much for me though. A Song For The Lovers was good though.