In Act 2, Scene 6 of Romeo and Juliet, why does Shakespeare not spend time on the marriage and write about the wedding in detail?
Shakespeare does not spend time on the actual wedding ceremony because the fact that Romeo and Juliet got married is actually far less important to Shakespeare than the reasons why they got married. We know that the wedding ceremony actually does take place because, in his final lines of this scene, Friar Laurence declares that the couple shall absolutely be forbidden to be alone together until the ceremony is completed, saying,
Come, come with me, and we will make a short work;
For, by your leaves, you shall not stay alone
Till holy church incorporate two in one. (II.vi.35-37)
Friar Laurence says this because he fears that the couple will commit the sin of fornication, which brings us to the reasons why the couple wants to marry, which is Shakespeare's focus. The couple wants to marry simply because they are young and lustful. We see a great deal of evidence for their lust in this scene. It has been frequently interpreted that when Juliet enters the room and Friar Laurence says, "Romeo shall thank thee, daughter, for us both," he is saying it because Romeo is actually kissing Juliet before the friar, and Juliet returns the kiss when she replies, "As much to him, else is his thanks too much" ("Romeo and Juliet," shakespeare-navigators.com). All of these premarital kisses before the friar show that they are lusty and unable to control themselves, even before the holy friar.
Another passage that demonstrates their lust is spoken by Romeo who refers to their "joy" and the sweet "breath" that they share and further says,
let rich music's tongue
Unfold the imagin'd happiness that both
Receive in either by this dear encounter. (27-29)
The reference to "music's tongue" and the "happiness" they'll share can be interpreted as again referring to kissing, and it can also be interpreted as having sexual connotations.
But most importantly, Shakespeare wants to use this scene to point out the dangers tied to their youthful, violent passion. We see this when Friar Laurence warns,
These violent delights have violent ends
And in their triumph die, like fire and powder,
Which, as they kiss, consume. (9-11)
In other words, Friar Laurence is warning that the violent, passionate love they feel is likely to also end suddenly and violently, which can be seen as foreshadowing their upcoming deaths. Friar Laurence is warning that, just like "fire" can be put out with "powder," their passionate, fiery love can also die very easily and suddenly.
Hence, we see that Shakespeare's main focus in this scene is to point out the uncontrollable lust the young couple is feeling and to allude to the dangers of violent, uncontrolled emotions, even love and lust.