I don’t think that’s such a big difference. Of course, the bigger the need, the more a posek would be willing to rely on an opinion he wouldn’t ordinarily rely on.
The biggest factor here is that the liquid medications are flavored, and their consumption is considered eating, as opposed to pills, which, although not without issues, are more lenient.
The problematic ingredients in liquid medications can be the flavoring, but often the problem is glycerine, which can have either a vegetable source or an animal source. The producers use either, depending on price, which changes.