I'm not sure the context of the question (episode please?) but I don't think it's necessary to answer this. I've studied media and film for a while now, so I think this insight will be of some worth regardless. Ash may not remember the legendary birds, or any Pokemon in the series for that matter, due to:
Plot convenience or plot device. It may be helpful to the writers if Ash is not familiar with certain Pokemon, as this can segue to a scene or exchange of dialogue that may not have made sense otherwise.
The Pokemon anime's target demographic is children, and children do not care about continuity of characters or about what makes sense, so long as they are engaged. The writers and producers of the show can therefore sacrifice continuity in order to increase their appeal to children. If this means sacrificing logic, sense and consistency -- values of a demographic older than the show is written for -- then the writers will absolutely go for it. Children's entertainment isn't the same as other entertainment; that's not to say you can't enjoy the Pokemon anime, but you can't expect it to flow like more mature content.
If Ash were to remember everything and grow from everything, his character would develop beyond what would suit the show. Ash's character is supposed to be relatable; if he knew everything already, then viewers who aren't experienced with Pokemon or the show already will not find Ash enjoyable to watch. They may wonder why the characters in the show treat seemingly-foreign concepts as common knowledge; remember the show is primarily aimed at children, so any confusion or ambiguity will be very disparaging. Newbies to Pokemon cannot align themselves with Ash if he knows everything -- if the "beginner" audience does not enjoy the show for that reason, then it has failed to do anything but please the minority group of viewers who have watched many previous seasons, which is not sustainable for the show in the long term. Same reason Ash never ages; children can't align with a 30 year old adult in the context of the show.
Reintroducing concepts can reestablish or familiarise viewers with certain concepts. As above, the Pokemon anime's target demographic is low and reasonably narrow, so viewers will come and go as years go on. As a result of this, continuity between different seasons and movies is not likely, as this may confuse and alienate young new viewers who go in without context or prior knowledge. The writers cannot expect that their viewers have seen previous seasons, because they would require a higher viewer demographic that what the show is intended for, fundamentally. The simple reality is that if you are not a child, the anime wasn't written for your demographic, so you won't get the same experience with it as you would if it had been written for your demographic. Long-running shows and movie series re-establish key concepts or ideas all the time, so that nobody is left behind and the events can be followed properly by anyone. Pokemon is less subtle about it, because again, kids don't care if it's not subtle.
Case in point, if Ash does not know what a Pokemon is and checks his PokeDex as a result, then the writers are simultaneously answering the curiosity of its new viewers and potentially setting up for new dialogue to progress the plot (as above). Additionally, periodically checking the PokeDex in the show can help advertise the Pokemon; if writers can properly flesh out a character or Pokemon that appeals to the audience, then viewers may return to the show later, or buy merchandise or an actual video game. Remember this is as much business as it is entertainment.
If you are old enough to look at the show critically and question concepts like this, then you are already beyond the age that the writers will make reservation for with regards to the consistency of characters. That age is probably very low, and maybe that's a criticism you could make of the show, but it is what it is.
