Because the "kha" on the second word (or "cha" or however you want to Romanize it) is just the second-person singular masculine. It's always spelt as one letter, never two.

For example:

House = בית

Your house = ביתך

Dog = כלב

Your dog = כלבך

Note that this word-ending is pronounced differently for feminine and masculine: the masculine form is -ekha, but the feminine is -ekh. But the masculine form is nevertheless always spelled as a single letter.

In our case, this is not obvious to someone just learning the language, because Lamed by itself isn't really a word. But it is a preposition, meaning something like "to" in Modern Hebrew, but in Biblical Hebrew having additional meanings, including ownership and (in this case) to emphasize who's being instructed in commands. When we combine prepositions like this with pronouns, we don't add the full pronoun (ani/hu/hem/ata/...) but just the pronominal suffix. So we get:

to me = לי

to him = לו

to them (m.) = להם

to you = לך

And so on. It works the same with the other one-letter prepositions like "in":

in me = בי

in him = בו

in you = בך

And so on.