The guitar has not been a prominent instrument within Western classical music very long in comparison to instruments such as keyboard instruments, cello, and violin. Indeed, the guitar can be traced only as far back 16th century Spain. For reference to other instruments at this time, Johann Sebastian Bach was composing his renowned keyboard, organ, and concerti at the same time as a rudimentary guitar-like instrument was first being formed. It is because of this that guitar composition and performance was underdeveloped and not taken seriously for many years. Bach and his contemporaries and predecessors did not have an opportunity to compose for this burgeoning instrument, and later composers such as Mozart and Beethoven did not hold the guitar in as high of a regard as other well-established Western instruments (Encyclopedia). One cannot blame them; the early guitar was crude, used only four strings, and made of only wood (Encyclopedia). It was also used in a traditional folk setting that appeared to stand in contrast with music of the high-classical style. If the guitar would be taken seriously in any regard, extensive improvement was needed to gain legitimacy.
Instrumental development gained momentum from the late 16th-early 19th centuries (EB). First, a fifth and, later, a sixth string was added for a wider range of notes. The string materials were also changed from being made from animal intestines to metal. More complicated and elaborate works were now possible; however, this increase of possibility was not fully taken advantage of until the 19th century (Martin). This can be best demonstrated by notation style of the time: from the 16th to the beginning of the 19th century, guitar music was written almost exclusively in tablature and chord symbols (EB). This style of writing does not allow for exact rhythmic and melodic notations to be conveyed exclusively on paper. Instead, this style of writing only allows the performer to understand the overall form and structure of the work and relies on previous careful listening of the piece to understand the overall piece. This slightly more rudimentary form of musical notation was more attainable for amateur musicians but also, again, prevented classical composers from writing for the instrument. However, the 19th century saw a transformation from tablature to traditional staff notation which opened the door for classical guitar writing.
Image: Save Me, ChinoTenshi, 2016 (MuseScore). This image is modern tablature, which is a shorthand way to notate music. Tablature of the era was very rudimentary and poorly documented, so this image is a modern rendition of tablature. Such notation is still used frequently today.