Why didnt the king of england,scotland and ireland declare himself an emperor?

As a supplement to that linked answer, and at the risk of stepping on our Medievalists' toes, one thing I would add is that the question would mean very different things depending on the period.

If we are in Medieval Europe (which is actually a fair starting point, because the lands ruled by the Angevin and Plantagenet Kings of England were re-styled as an "Empire" by 19th century historians) part of the reason, say, Henry II didn't just up and declare himself an emperor is that, well, that's not what Christian rulers did. Europe was supposed to have one emperor, who in theory was following in the lineage of the Roman Emperor, and who was crowned in this role by the Pope. This started with Charlemagne's coronation as "most serene Augustus, crowned by God, great and pacific emperor, governing the Roman Empire" on Christmas Day, 800 by Pope Leo III, and gave rise to what would be known as the Holy Roman Emperors (ruling, of course, the Holy Roman Empire), who continued until 1806, and more on that bit in a minute. Just as a note, quips by Voltaire to the contrary, the "Holy" did actually mean something, and in this case it indicated that this ruler was governing with the blessing of the Church, as given by the Pope (as if the Papacy and Emperors never fought with each other, ha!), and held a paramount position in (Western) Christendom.

Now note that "Western", because there was the small issue of an actual Roman Emperor still ruling all the way to 1453, only it happened to be in the Eastern Roman Empire (which again 19th century historians have redubbed as the Byzantine Empire...frankly you'll see that a lot of our understanding of history gets shaped by the 19th century). Now, these emperors of course had thoughts about new upstarts claiming to be Roman Emperors in the West (and in their defense, Odoacer had actually sent all the symbols of the Western Empire to Constantinople after deposing Romulus Augustulus, so that whole institution was defunct in their eyes), all the more so because it was claiming authority from the Patriarch of Rome, who in Constantinople's eyes was also claiming a paramountcy of authority among Patriarchs (there were four more) over the Church. To skip ahead a bit, ultimately there was a schism in 1054, so Constantinople had its own (Eastern Orthodox) Church to legitimize its rule.

So what changed? When did we start getting multiple European emperors? Or to be more specific, multiple widely recognized emperors ruling their own empires?

Well, we'll have to start with Russia. Or more specifically, Muscovy. The rulers of Muscovy were Grand Dukes, who at various points after the fall of Constantinople in 1453 claimed to be the "Third Rome" (after Rome and Constantinople) and to be defenders of the Eastern Orthodox faith, all of which helped them in their claims and attempts to rule all of the former (Kievan) Rus. What helped them in this was that Ivan III of Moscow had married a Byzantine princess, Sophia Palaeologina, and so Muscovite rulers after that time had some claim to belonging to the last Byzantine imperial family. Ivan IV, Sophia's grandson, who the English-speaking world knows as "the Terrible", went that last step and had himself crowned as "tsar" (coming from "Caesar") of all Rus in 1547. So here we have an emperor again. And in case this was too "Oriental" or weird for the rest of Europe, Peter I just went the last step in 1721 and renamed the state the "Russian Empire" and his title as "Imperator".

Of course, non-Orthodox Europe still had a Holy Roman Emperor. Why wouldn't England pull a Russia, especially after the break with Rome? Well there are a few reasons here. One, of course, is that the idea of being an "emperor" like the Holy Roman one is still kind of a super-duper Catholic idea, which wouldn't really have flown in 17th century England. Another is that being king of a number of kingdoms is not quite the same thing as being an "emperor" - the King of Scotland, James IV, became King of England in 1603, but this was a "personal union", similar a bit to how Canada and the UK are today - both countries share a monarch, but otherwise had their own legislatures, nobility, law codes, etc. Ireland similarly was its own thing, with its own parliament, and while the Papacy had acknowledged English kings as Lords of Ireland from the late 12th century on, the title "King of Ireland" was created by the Irish parliament for Henry VIII in 1542 (so this actually was an upgrade in title). Scotland remained its own kingdom until 1707, and Ireland until 1801.

So why not declare the whole deal an empire at those points? Well, beyond the reasons noted above, an added wrinkle was that from 1714 until 1837, Kings of Britain were also rulers of Hanover, and those not only subjects of the Holy Roman Emperors, but one of the electors of the Holy Roman Emperors (happy Electoral College day, by the way). So declaring themselves an emperor in their own right would be a little awkward.

So what changed with the 19th century? In one word, Napoleon.

The French Revolution and the wars it unleashed in 1792 turned upside-down not only France, but the rest of Europe, including the wide swathes of the Holy Roman Empire that were conquered by Revolutionary armies, who set about dismantling the ancient, traditional structures of power. Eventually Napoleon rose to the top of France, ending some of its more revolutionary tendencies and consolidating his own power through a recreation of a monarchical government, albeit one that distinguished itself from the ancien regime. To this crown this achievement, if you will, he crowned himself emperor in December 1804, and here I literally mean he crowned himself emperor, as memorialized in this painting by Jacques-Louis David. Note that Pope Pius VII is present, somewhat dejectedly giving a blessing symbol, but isn't doing any crowning - Napoleon crowned himself, and is busy crowning Josephine as Empress. This was something of a controversial and revolutionary break in itself, that a monarch could just go ahead and declare themselves Emperor, just because, and ironically it required the revolutionary upheaval that Europe experienced to even make such an idea possible to monarchs - conservative ideology and politics was in many ways just as revolutionized after 1789 as revolutionary ideology and politics was. Before then, one just didn't crown themselves emperor, because it just wasn't done, and why would you want to pretend to be the same as this odd German ruler anyway?

Of course that Holy Roman Emperor was swiftly no more - Napoleon abolished the office (and empire) in 1806, and the Hapsburg rulers of Austria, who had more or less been the Holy Roman Emperors since late Medieval times, re-styled themselves as Emperors of Austria. So now we have a Europe with three emperors, and no real external authority or tradition to say who should or shouldn't be an emperor, beyond basically individual rulers themselves. Europe would give up a French emperor in 1814, get him back briefly in 1815, get rid of him again, then get his nephew back as Emperor in 1852, ditch him in 1870, and then get a German Emperor in 1871 instead. By that time, if even Prussian rulers could be an emperor, why not get a similar title for Queen Victoria?