Depends what you mean by "in Web".
XSLT is very widely used. As far as we can judge from metrics like the number of StackOverflow questions, it is in the top 30 programming languages, which probably makes it the top data-model-specific programming language after SQL.
But XSLT isn't widely used client-side, that is, in the browser. It's usually used either server-side to deliver content on demand in response to HTTP requests, or it's used in batch mode as part of a publishing workflow. It's also used, of course, in many applications that have very little to do with the web, e.g. in print publishing.
There are a number of reasons XSLT isn't extensively used in the browser. The main reason is that good conformant XSLT support was very slow coming from the browser vendors; no-one wanted to use it until it was available on every browser, and by the time it was available on every browser, the things people wanted to do in the browser had moved on (remember "Web 2.0"?) and the XSLT implementations in the browser didn't help you build interactive applications or fetch data using AJAX.
The other issue is that XSLT is a love-it-or-hate-it language. Its declarative, rule-based, functionally-oriented paradigm appeals to many because of its high-level nature, but can be off-putting to those whose only experience of programming is to write imperative code that tells the computer exactly what to do and in what order.