Primarily it’s because the people providing help and assistance here should not promote a personal preference towards libraries. This is why you don’t find people pushing YUI or Dojo or MooTools etc. If anyone was to do so, there would be umbrage from the general community, and the same applies to jQuery too.
There can also be times when you may want someone to realise that what they’re wanting to achieve may be easier to do by using a certain library, or external resource. By all means mention that, but if you do then you must approach that idea with the understanding that they may not want to take that idea onboard.
If someone says that they are currently using jQuery, or some other library, that then is a good time to start applying techniques from that particular library.
In all fairness, the library isn’t that big. Version 1 is 90KB and version 2 is 80KB.
However, with jQuery, it’s important to note that version 2.x has dropped IE6-8 support. Version 1.x still has IE6-8 support.
Here’s a comparison of the various major frameworks that are available: