Such an adjustment for the requirement to claim bonus code is a common thing in gambling business.
It is lower in the beginning to attract more new players and increased a bit from time to time, as it is also part of marketing strategy IMO.
You should not take it as a burden, you are not forced to reach the wagering requirement all the time.
Promo code like this is not something to hunt, it is a bonus when you are playing in the site.