it's simple:
1) their phone OnePlus 5T just recently released, not too long ago. So they can't lower the price even it's their 4th birthday.
2) their phone is already very cheap (very low price), and they have very low profit margin compared to other flagship. They can't lower their profit further.
3) by theory, the oneplus 5T is supposed to be more expensive than oneplus 5. But they decided to remain the same price for oneplus 5T to maintain sales since they know oneplus 5 users will hate it. Therefore, they have already "lowered" the price for oneplus 5T to make it the same as oneplus 5. So, they can't further lower the price