1) The offers look very good, so you register. Only then do you become aware that things were not what they seemed, usually because the advertising was misleading, or made exaggerated claims.
One example is the Microgaming "free money" offer. This is usually "get $2000 absolutely FREE, and get to keep your winnings".
At first sight this looks very good, and whatever you win, you keep, even if only as a bonus.
Having registered, you then discover that you can only "win", say, $100, and that it has to be wagered 30x, AND you have to deposit. Thus, a fair bit of the initial offer is not actually true.
Having got this far, players may feel they have been "set up" for a scam, so will NOT go on to deposit, even if they do end up playing their free money for "shits & giggles".
Another common example is the introductory free chip. This is usually a small amount, sometimes $10 or less, and MOST players end up losing it all.
A few however, just get lucky, and manage to win enough early on to be worth withdrawing. This is when they experience the operator doing their best to NOT pay out on the free chip, often for reasons not mentioned in the initial offer, but that are buried somewhere in pages and pages of "smallprint". This makes the player think the site is a scam, and that every time they win, they will face a battle to get paid, thus they will walk away, rather than risk real money with a first deposit.
Of course, you have those players who make a career out of playing ONLY the introductory free chips, and take any winnings generated. This group never intended to deposit anyway, so there is nothing the operator could have done differently to convert such players into regular depositors.
Apart from these, there will be players who fully intended to deposit, but either they find that for some "technical reason" they just can't, or they become aware of some negative information that scares them away in the time between registering and putting together their first deposit.
Experienced players tend to check much of this out before they even register, and will simply not register if they don't like what they see, so the operator has no idea that they have put these players off.