After giving a bounty of 250, why are lower value bounty options removed?
It is better to keep all bounty values, even after a bounty is given once.
That is how the bounty system works.
From the FAQ entry on Bounties on main Meta, How does the bounty system work?:
Why is the system requiring me to choose a bounty amount that's higher than the minimum?
For most questions, you can start a minimum bounty of 50 reputation. There are some cases where the system may require a higher amount, though:
If you have already answered the question before, the minimum bounty offer is 100.
If you have already offered a bounty on the question before, the minimum offer is double your last offer (see below).
Note that if you offer several bounties on the same question, you will have to double the amount each time (or more). That is, if your first bounty was worth 50 reputation, your second bounty on the same question will have to be for at least 100, your third for at least 200 and so on. If you've already offered a bounty for more than 250, you can still offer more bounties for 500 (the maximum amount) as long as you like (or as long as you have the rep). This doubling applies only to bounties by the same user on the same question.