I think that topside should always have a lower population in general, but here are a few reasons why I've moved away from thinking about topside characters (as someone who has played some in the past).
1. Lack of corporate competition - It seems to mean that topside lately has been less about bolstering one's own corporation and more about promoting a general pro-corporate theme. Of course, there are plenty of things that go on in the background that most are not aware of, but to me, as someone who is attracted to conflict RP, having more in-your-face tension between other corporations/running operations against them/awkward scenes of competition would make the environment more attractive.
2. Economy - For most players in the Mix, moving topside requires a financial sacrifice, at least temporarily, where they end up making less money. Especially for a player who is not paying for a membership pad, sources of automated income are much, much more difficult to find topside, and, in the end, you ultimately end up making less money. Over time, the prospects of that may improve with certain archetypes that can produce products for sale, reimbursements, etc., but in my experience topside does not really bring the sort of wealth that people desire - at least immediately - and the makes people not to want to take the step.
(I think there are places where the automated income rule could expand topside better, but that might be revealing too much IC information.)
3. Reputation - Topside has a reputation for being a place where people go to idle, play slice of life, and usually engage in relationship/sexual RP. This tends to make people like myself, who don't engage in that sort of RP, hesitant to join. In addition, topside simply moves slower - for good reason - but most of the time I log on, I want to either find a bar that I can RP in, or do something with some chums. That's not very possible.
4. Rose-colored classes of Mix: This is likely one of the biggest reasons. The Mix is a slum. You can find threads about it throughout this forum. But, if you view SIC, or a simple bar chat, many people are profoundly unserious about the dire straights about where they live. If the Mix became more active, violent, and actually dangerous you may find more people moving topside. This is not the direction the current playerbase is going.
5. Lack of available topside mentors - I've had the experience a couple times in the past where veteran corporate characters simply could not be bothered to mentor young players on what is available at their disposal. Makes me wonder if there could be some sort of corpie-WCS position that could run through the gamut.
Generally, one of the themes of SIndome is that it takes time to advance, at least in IC jobs, and, if you have to spend that time RPing with only 2-3 people outside of major events that happen once every three months, it gets very easy to see why the attrition rate topside is so high.