Can't join a tournament

I'm currently 1490 points in Bullet and I can't join a <1500 arena bullet tournament (I've got also more than 20 classified games)
Anyone knows why?

Might be because you have been over 1500 before.

yes, because you've been over 1500 in the last week

That is still incomprehensible, because his current rating is less than 1500. He should actually be able to participate. What do his old ratings have to do with it?

Ok, so let me just lose 200 points on purpose to enter the tournament and then beat them easily.. Why shouldn't I?

thats correct, if you ever reached that rating it means you have potential to play on that level and win with those peoples with 1500 rating point. Lichess is doing good job to preventing sandbagging.
I remeber that even Kasparov after his retirement wrote on his site that he is now playing only simultaneous exhibitions, but if you have ever reached 2000 elo he will not play with you in simul. So even he agreed with that rule.

Thank you all very much for your answers!:)

or you could have not played enough games

If they didnt change the rules, then your rating has to be under 1500 for one week. See @bufferunderrun's answer here:

lichess.org/forum/general-chess-discussion/help-regarding-tournament-rules#3

This could get documented in the tournament faq:

lichess.org/tournament/help?system=arena