Churches probably have some kind of theological explanation, but alternately kneeling, sitting, and standing is much better physiologically. Having a large and diverse group of people all stay in the same position for an extended period is very rough on their bodies, and some more than others. Periodically switching prevents injuries caused by maintaining too much static load for too long in any one place. Probably also helps keep everyone awake. For the same reasons it's a good idea to occasionally switch positions while working.

it is all things but diverse

https://en.m.wikipedia.org/wiki/Ethnic_groups_in_the_Middle_...

There are theological explanations but it is also explicitly taught that physical movement and awareness of your body is an important part of prayer, similar to how bells and incense ask you to include those senses.

Also there isn't a rule against sitting, orthodox churches have seats for people who need or want them and it's absolutely normal to see people sit for some or all of services. It is discouraged to notice who or wonder why.