FOr me the reason why I suggest them is because tiels tend to be fun, active, easy, low up keep (in comparison), and hardy birds. If you buy from a reliable breeder they can be easily tamed and life long friends. Also their bites are less severe than others and tiels are laid back enough that they can be a good option for young children. All of the above is a comparison really. If you put a tiel and a conure side by side the two species are night and day. A tiel will put up with your fumbling while a conure (most of them) will simply tolerate it for a time and then push you into the direction that the bird wants to go. Also their noise level is usually managable. Tiels scream for various reasons (as do all birds) but their personality allows them to be trained to replace screaming with other sounds like purring or wooping or words which is a huge plus. Tiels ultimately do not require the time that another larger or more pushy bird would need. I think a lot of bird owners go into things with the idea of getting a bird who talks , who doesn't make a lot of mess, who doesn't scream all the time, who is cute and adorable and does cute and adorable things (just like on youtube!), doesn't bite, and assumes that every bird they come by is going to meet those standards. Truth be told the best talkers are parakeets (its true), the most entertaining (IMO) are conures or caiques, the only bird that doesn't scream is a stuffed one, biting is a going to happen I don't care how well trained a bird is, what is on youtube are the exceptions not the rules, and on the subject of cute and adorable... cockatoos do it for me but so do most other birds too. Sorry for the rant. Plain and simple tiels would be my suggestion to people because tiels can meet most of their expectations (males) and in the sad case that the potential owner decides that birds aren't their thing tiels are easily rehomed and easily adopted at rescues. Its a sad truth, but one I think it's wise to look at. Not to say that every newbi bird owner is like this, I know I wasn't and neither were any of you (i think). I'd personally rather see a rescue filled with tiels because of the lower cost of vet care and boarding and general upkeep than a rescue filled with large parrots who bankrupt the business and are generally harder to rehome. I think owning a large parrot is a responsibility and a privelage and should be entered into as such.
Sorry for the rant.