Reworking something of an older answer

Depends on the situation, the country, the time, as u/Temponautics points out in the linked answer, titles have to be seen in their context and the local history.

Conquering the land and being the dominant regime might help but one could be the most powerful man in all the land and not become Emperor but seek power via a different route. One could be a small regime of one province and a depopulated mountain range and lay a claim for Empire. China's Son's of Heavens are not the same as, say, the Holy Roman Emperor and rules for who could be Emperor could change even in a country to reflect realities of the time. Your question suggests you are more looking at a European situation.

Someone wishing to become Emperor might have to look at the situation they were in: what was the position with the last (or still existing) dynasty? What was their position abroad and at home? What were the traditions and expectations? What was the value of what they plan vs the risk? What legitimacy arguments could they make, what signal did they wish to send and what might it end up sending, and so on. Then hope they get that judgement right.

So what stopped a ruler from declaring themselves as an Emperor?

One should acknowledge that even absolute rulers of powerful nations, Emperors themselves, were not free to do whatever they wished. At least, not without consequences that they need to consider when making a decision. A ruler, great or small, lived among structures, among cultural attitudes, among situations and people, all these rulers might be able to shape and influence but they couldn't always escape it.

A strong ruler could make a decision that might be unpopular if they were determined to push something through, against the advice of officials and amidst protest, but there would be consequences. That could simply be burning a lot of political and/or diplomatic capital or helping destroy a dynasty. Power is limited and power can be taken away if one is not too careful.

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Dangers Abroad

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A count does not rule as the sole ruler in their world without neighbours, even if they had no fealty to another. Powers you trade with, perhaps religious authorities, allies ranging from close to uneasy, powers that your wary of, perhaps neighbours your able to dominate, perhaps some that consider you under their influence.

Becoming an Emperor isn't just a name change, it is a change of power dynamics and authority. For those lesser powers or those once equal to you, perhaps you are hoping to use your new status and the honours you can grant to woo them. There is a risk however that your elevation puts their nose out of joint, they are still a count and now you are a fancy Emperor, is this a slight on them? Does this act challenge their authority at home by raising questions about legitimacy? Do they feel you are asserting rights and authority over them?

If a surprise or they had warned against this move, they may question how reliable an ally you are and whether it might be better to switch alliances to protect themselves from impending disaster. Or to avoid tarnishing their name. Without alliances, you risk being vulnerable and your lands relying solely on your own resources and power.

For those there were hostile or had been eyeing your lands, your rise may offer opportunity. Strike while things are uncertain, while your lands and allies are unsettled, and see what happens, an early defeat for the new Emperor will... not look great. Depending on how the rise to Emperor was handled, your ascension may also provide a ready-made excuse to attack and be the good guy while invading you. Treason for example.

For those who deemed themselves superior to you (including King's and Emperors), this is a potential challenge to their authority. Circumstances may mean they will have to bite their lip and store it away for another time but they might also try to assert their authority by force. Even at lower levels than someone claiming to be Emperor, a junior partner's rise could create unease and a need to yank their chain. This goes far deeper than that, this is very likely a break-away and assertion of your independence, that you owe no fealty but are equal or superior to them.

If you can do it, who else among their ranks might be tempted to self-declare if they let it pass? If they let this pass unchallenged, what will the repercussions for them on the wider among other powers who see them not defending their own interests? What does this say about their claims to legitimacy and authority?

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Local Concerns

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Who wouldn't want to serve an Emperor? Potentially quite a lot. They may be concerned at the fallout abroad and home, that this act might be dangerous for their land and themselves. They might not consider the act moral or appropriate, it raises questions about the counts moral character and judgement. It may go against local traditions and a sense of their own history, their culture. It might be that your leading families and officers have connections with other powers, split loyalties that this move will wrench and force them to pick a side. Or that not all your subordinates are without ambitions, that such a move might allow them to wiggle free of your grasp or seek your old position for themselves. Or someone who feels their own position might be placed in danger by your power.

There are a lot of things your leading families, officers at your court and guests could do. Private protests and if that didn't work, go public via resignations (or refusal to serve in your new ranks), words in court, memorials or whatever the society of that time has to argue this is inappropriate. Arguing against your legitimacy and in their actions, embarrassing your new court with word likely to spread about the feelings expressed.

You may well be relying on leading families for all sorts of things, access to resources via taxes and other means, manpower for wars via their raising troops, for public support. Your empire needs funds, it needs troops, it needs officers and ministers to carry out their duties and if they quietly sit at home and neglect the court or the demands, you have a problem dealing with challenges of rule.

Even worse, the families could side with an enemy. A rival contender of your bloodline (or a former ruling bloodline) could seek to muster support among those concerned. A leading figure who could use your new rise as a contrast, play into traditions of the area, their modesty and restraint. An invading power who has connections, a King they feel more loyal to than to yourself when push comes to shove.

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Legitimacy

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Rulers, like all people, are born and shaped by the society they live in, the rules and the expectations. As they rule, they will be (even if things are in turmoil) some sort of system and expectations of how things are or should be. They should have some idea of what will be accepted or what would simply be dismissed as absurd.

People will hold their position via some sense of legitimacy as well as the ability to hold power. Family tradition in the area perhaps, old alliance to another power or appointment from a monarch? Perhaps a new count via force of arms and ability to rule? Undermine that legitimacy, undermine the authority to hold that position, leave openings for internal and external enemies. Legitimacy won't save one from a spear to the chest but it is not without importance.

To be an Emperor that lasts requires the rule and change being accepted on some level. The rules may depend on the time and the place but if declaring oneself an Emperor sees one's support at home collapse and invites invasion from without (or rebellion from within), it is a problem. What purpose would that serve?

Deciding to become Emperor sends a signal but what signal is one aiming to send and how will it be received in practice? Not just in your own time but in history to come. Do you wish the legacy to be throughout history to be "became Emperor becuase he wanted to" or to carefully build a narrative that will last beyond you, saying why you and your family deserve to be Emperor? Providing legitimacy beyond your life, shaping your reputation at the time and trying to shape a reputation for those that come after?

What is the argument for being Emperor, as a justification that people then and since would listen to, but also what is the practical case on the ground to make such a change? Without those, with the risks that come from making that change, one is taking a risk.

Please refer to a similar question and answers by me and u/dongzhou3kingdoms here.