Why is Giratina the only level 5 boss in a raid when in level 4 and below have multiple pokemon?

Raiding is the only way to get a Giratina (don’t trade for a Giratina if you don’t have a Giratina). Whereas I can get 5 level 30-35 Houndooms in a day and never even do a raid.

Level 5 bosses usually take multiple trainers, and take prep-work to take down. A level 4 can be duoed by many trainers, or trio’ed with decent trainers. So 2-3 people together should be able to take down any 1-4 tier boss. However let’s say the level 5 egg contained either a Groudon, Kyogre, Raikou, or Ho-oh. Even with 6 sub-optimal trainers you are going to have a hard time. With one boss, everyone knows what the egg will hatch, and knows what they need to counter it. You have time to prep and get the right team.

Granted Giratina is a weak raid boss that can Trio’ed fairly easy, without the right counters, people would need 4/5. So once you figure out and power up to that Trio level, you can drive around and raid down a few level fives with a couple friends as a great time together.

For the most part, there's always only one T5 boss at a time. There has been some overlap at the start and ends of some runs and there's been a couple events that have had multiple at once but each legendary generally gets a month to itself. Previously we had Mewtwo from September 20th to October 23rd, now we have Giratina until November 20th. No one knows who will follow though it's likely to be either Palkia or Dialga.

Part of the reason we only get one legendary raid boss at a time (typically for a month) is so Niantic can stretch out new content. If they released multiple at once (or, Arceus forbid, all of the Gen 4 legendaries at once), people would go heavy on legendary raids for a month or so, and then people would be bored until Gen 5. Releasing them one at a time helps spread content out.

In addition, there's some inherent issues that crop up when multiple legendaries are out at the same time. For example, if we had Dialga and Uxie out at the same time, the following are likely to happen:
• people raid Uxie once for the Dex entry and never again. Those who missed out on that first rush have a much harder time getting one.
• people show up to a gym with a lvl 5 egg hoping it's Dialga, but Uxie hatches. Those who don't want it leave, which may result in those remaining lacking the numbers for it.

Say we have two desirable legendaries out at once. Dialga and Rayquaza (with shiny version available):
• Lots of raiding happens, people get burnt out, and Niantic ends up with 2 months worth of desireable content packed into one month.
• People probably end up doing fewer Dialga raids than they would have if it was the only tier 5 raid for that month, because there's likely going to be a bigger push to raid Rayquaza because shiny.

Short Answer: Lvl5 raids are reserved for the legendary pokemon of the month, lvl4 and lower include normal pokemons that's why there is more variety. Having only one legendary avaible helps Niantic keep people hooked and coming back to the game ebery month, specially if it's a new legendary release