Your general assumption is correct, and you can use than with preferable if you make a slight change to the sentence:
Lemon juice is more preferable than tea.
However, even though that's grammatical (syntactically), it's still quite unusual, simply because it's not normally used nearly as much as preferable to.
From Merriam-Webster's definition of preferable:
: having greater value or desirability : being preferred
By the same token, compare the following sentences:
✘ Lemon juice is desirable than tea.
✔ Lemon juice is more desirable than tea.
The words preferable and desirable do not, on their own, convey an explicit sense of comparison between two things. They are adjectives that can stand on their own, and you can use either without adding any kind of direct comparison at all:
Lemon juice is preferable.
Lemon juice is desirable.
The use of the preposition to after preferable is a particular case of comparison that doesn't always work. For instance, it does not work with desirable in this case:
✘ Lemon juice is desirable to tea.
Note that this is actually an error of semantics rather than syntax, because the following would be acceptable:
✔ Lemon juice is desirable to Jane.
→ Jane desires lemon juice.
But if you add an adjective that invites comparison (such as more) to the words, then their use with than becomes normal.
Unfortunately, any of these specific examples are difficult to quantify with general rules; there are many exceptions where even though the syntax seems correct, the semantics don't allow for it in idiomatic use.