It pushed them out ahead of stalwarts like Cognos and Business Objects for a while.
When Microsoft entered the BI landscape they changed the world as we knew it by bundling a fairly decent BI stack right along with MSSQL and native Excel connectors, for free.
Many big names in the enterprise space went for it (and subsequently choked on Sharepoint) but I think this hit Qlikview pretty hard. They didn't have the legacy to withstand that battering that many of the more established BI offerings took during that chapter of the story.
Good riddance.
Qlik Sense is pretty snappy, has much improved UX, etc.
Qlik are top-right quadrant for BI tools, on a par with Tableau.
QlikView does have disadvantages (mostly related to UI as you rightfully mentioned) but its performance is almost never among them.
I hate their insistence on natural joins.
Your website gives a decent rundown, only part of it isn't displaying in portrait orientation on my iPhone6s+ :-(
I personally dislike the fact it relies on natural joins so you are forced to use defined columns with the same names and use Qlik syntax to disable table linking, but others love that feature.
As an ETL tool actually it's pretty good. But if they had decided to go with straight SQL and extended it to external sources then it wouldn't have stupid syntactical EXISTS and NOT EXISTS clauses, but that's all I can really fault it on.
It's a good tool for what it does, and it turns out that what it can do is a lot! The biggest issue I found was that whilst I picked up the syntax basics in a day and it's more intricate features in a week or two, most of the Qlik developers in Australia are morons who charge a lot and deliver very little. I was very lucky to have an amazing colleague who does know his stuff, but the consultants who we were eventually forced to get in were awful. Then again, the CEO of that company was just walked out the door for gross incompetence so it was probably a combo of stupid management and a limited talent pool for Qlik devs.
It was fun because it was a puzzle and I was helping fix broken things, but at some point you just ask yourself "what for?" when things are so broken and improve very slowly.