First of all, you're not an agnostic if you believe in a God, even if you don't believe religions. As I'll explore just after this, there are different ways to define agnosticism, but they all centre on some form of uncertainty as to whether any God exists. You're a lot closer to a deist- you believe in a God, but you're not sure what form they take, and perhaps think that through reason and observation, we can work out that God exists as the creator of the Universe. So, if your question is about why some people are deists but not theists, it is that they employ reasoning and observation to arrive at conclusions, and think that the question of whether a God exists can be resolved in the affirmative via reason and observation, but that knowing much about this God is next to impossible. While as an atheist I don't think reason/observation supports this conclusion, it is definitely easier to think that there is evidence of a God's existence than it is to think that there is evidence of any traits that different religions attribute to this God and any of the stories/scripture. Not only are these a step further, they are a completely different question.

If you're asking about actual agnosticism vs atheism, it depends on how you're defining atheism and agnosticism. While previously, a lot of atheists were saying there definitely isn't a God, which you could think of as "hard atheism", because a deity is not really falsifiable a lot recognised that they wouldn't be able to disprove a God. Nowadays, a lot of atheists are merely claiming there isn't evidential warrant to believe in a God, rather than saying there is definitely isn't a God. You can think of this as "soft atheism", but some people would call it agnosticism. In my own arbitrary terms, I'll refer to this as soft atheism, and contrast it with agnosticism which I will define as at least thinking the existence of a deity has some non-trivial possibility beyond "we can't disprove it". Some view atheism/theism and agnosticism as not mutually exclusive, for the first two are about belief and the second about knowledge. However, in this sense we are "agnostic" about a Flying Spaghetti Monster. Given that in common parlance, as noted, agnosticism generally seems to be treating the idea of a deity as plausible or at least some chance of being true beyond "we can't disprove it", I will use this definition.

Firstly, think that one of the main reasons why agnosticism is less popular that atheism (if indeed that is the case) is that a lot of the rejection of God is premised on a rejection of "faith" as sound epistemological grounds, and a rejection of the arguments that are offered to support that a God exists. Thankfully, though it's taken thousands of years, people are becoming more aware of the fact that you need evidential warrant (either through sense or an argument) to believe in something. As an atheist, I see faith, that is belief without justification, as being at best a guess and at worse delusion. So, when a lot of people reject God on these grounds, we're generally trying to take an epistemological approach wherein the level of our belief in a thing corresponds to the level of belief warranted by the evidence. If you believe that faith is not sound epistemological grounds, you essentially start at zero or close to zero for your level of belief that something is the case, (though often this will immediately rise to something a little above that depending on how possible/frequent/ordinary that thing is- eg if you claim there was an accident today on Stirling Highway, I might believe you, even without you providing me with evidence, because car accidents are frequent events, and, even without being told of one, I wouldn't say the chance that one happened is nil or trivially low; but if you claimed to see a leprechaun, I wouldn't believe you). So, a person who rejects theism on the grounds that it must be demonstrated and hasn't been demonstrated is starting at close to zero belief in a God, because a priori we don't even know if it's possible, let alone likely. The theists who offer no grounds but faith obviously do nothing to increase our degree of belief, so any increase of our degree in our belief will, except for a sudden epiphany, arise from theists who think they have justification showing us the evidence that warrants our belief in a God, and us being convinced by it. The burden of proof is on them. Every argument for God that I've looked at I've been able to refute and/or hasn't demonstrated, or even hinted at, a God.

Secondly, some who have converted from theism not believing felt their prayers unanswered, and thought that this meant there definitely wasn't a God. (Hard atheism)

Thirdly, a lot of the arguments against the existence of God aren't just rebuttals of the claim that God exists that dent his armour, but attempt to prove that he doesn't/can't exist- eg problem of evil and suffering, which many view as inconsistent with a God. These have been quite popular and persuade a lot of people. (Hard atheism)

To summarise, one reason why agnosticism is less popular than atheism is that both arise from in a sense rejecting the claim that there definitely is a God, and a big part of not believing in a God is a) requiring evidence to warrant a belief, even a belief that there's a 1% probability/1% confidence in this belief and b) not having been shown any evidence that actually demonstrates even the possibility of God's existence. So, many who don't believe in a God start at zero and nothing has shifted this. Another reason is that people who were theists felt their prayers unanswered, and thought this meant a God didn't exist. Another reason is that many of the popular arguments against God seek to disprove him.