One big problem is that people often get glassy eyed when someone tries to tell them the facts, and then they see the costs, and believe they can do it better than a pro, or know better. I don't think there's anyone here that doesn't believe that a tankless system CAN work, but that many installations DON'T work well because of one factor or another.
There are situations where a tankless system needs enhancements to handle things a tank type does very well - take the example of a washing machine that fills and operates by short draws on hot water in another thread. The more complicated, the more maintenance and costs. Most people don't like compromises, and this means taking a shower when they want, and not having to wait for others...why have 2-3 or more bathrooms in the house if you can't use yours when you want? Sizing a system to support all users (and that is the code in many places) means (often) a pretty big inital cost.
Last thing, while a tank system should be drained, and with the new technology, the filter screen cleaned off regularly, many people never look at it after installation and it lasts many long years. You cannot do that with a tankless system. And, yes, you can get a tankless system to produce essentially unlimited continuous hot water if you size it properly. Most people don't want to do that either because of the costs and accept the limitations.
As to costs, standby losses aren't all that huge on a tank system, and the difference in equipment and maintenance costs plus installation on a tankless system, at today's energy rates, will likely make a tank system cheaper. A fairly typical tank system could be replaced 2-4 times over the cost of one tankless system. Plus, you can likely get your tank system replaced or repaired on a weekend, whereas trying to get a tankless repaired on a Friday night so you are not without hot water for the weekend or longer has a much lower probability of success.
I think many of the pros don't want to install tankless because people don't want to pay for a system that actually has the capacity they really need, and the installer doesn't want to deal with an unhappy customer.
As I've said before, you can get a tankless system that will work well, but, at least in my mind, it isn't worth the cost in most cases.