I have no idea so I'm gonna speculate. Most of the following is probably false.

bitrefill is probably the best known site for buying gift cards with bitcoin

bitrefill's card providers inform companies like walmart where their traffic comes from

regulators pressure companies like walmart to discourage bitcoin usage [this part seems unlikely to me so we're in conspiracy theory territory now]

walmart can show them they are taking efforts to discourage bitcoin usage by telling their card providers to limit purchases of their gift cards from bitrefill

walmart doesn't do the same to all btc gift card sites because they actually still want the business those sites bring them. They are just trying to appease regulators, and the easiest way to do that right now is to show them that they limit usage on the largest such site

Here's a theory that seems more likely to me:

bitrefill gets good deals on bulk purchases of cards by writing exclusivity contracts with some of their gift card providers

those gift card providers only get a certain number of walmart cards per month from walmart

walmart cards are very popular on bitrefill so if they don't limit them, their gift card provider won't be able to meet its obligations to other customers

and bitrefill won't just add a partner with more walmart cards because that would cause them to lose money since it would entail not renewing their exclusivity arrangement with their current provider, which is a very lucrative deal that allows them to buy cards at a discount but sell them at full price