Why does Jalter eventually get worse?

It’s just means AOE type servants will have better impact compared to single target servants, and it’s only in some parts of events. And since she is a single burst type unit

There's no single reason for it - but something to consider is that the early EoR period has a whole bunch of Anger-friendly Challenge Quests and boss fights. (plenty of Berserkers, Extra Class and high class diversity in a single quests). That makes her quite useful, but in the future more direct counters are possible.

Then there's the Quick Servants pushing their way in eventually, with proper support they are not as limited by cooldowns and basic card draws.

Bosses that do ridiculous damage even when you have class advantage.

Special gimmick bosses that are more about dealing with kill order, buffs/debuff, and triggering effects. Encounter design, etc.

Then there are of course a whole slew of AoE encounters, especially next year. DW making sure Merlin isn't always a great pick, etc.

Finally, relatively, Servants can become less powerful by virtue of others being buffed or having access to supports.

Anyway, it's difficult to pinpoint - but it's probably something we'll touch upon at some point. We haven't discussed it that much.

Break bars that single targets servants wont be able to beat on their own, AOE servants being favored to fight waves of enemies, Quick servants being relevant with the meta are enough to put her and the likes of kintoki and tamamo lancer at the middle/bottom of tier list

You can't one turn a single boss node anymore with her, and that's her main use for future event so there's that.
That aside, at least Jalter excel in rush mode where you can still OHKO a zerk boss or Ruler with double Merlin. She still is a very good stat stick so you just gotta have to use her carefully in the future.