Because completo agrees with volo, and volo is masculine singular.
Italian adjectives change form to match the noun they describe.
Examples:
Agreement is a very important part of Italian grammar.
It is understandable, but in everyday Italian, native speakers often say:
Al completo is especially common for fully booked / no seats left.
So while Il volo è già completo can be understood, many learners will also want to know that these alternatives may sound more idiomatic in real-life travel contexts.
They are similar, but the nuance can change a little.
For flights, hotels, and events, al completo is often a very natural choice when you mean sold out or fully booked.
So:
A rough pronunciation guide is:
eel VOH-lo eh jah kom-PLEH-toh
A few details:
The accent marks in è and già help show the correct written form, and they also point to the stressed vowel.