Commission rates are lower at my firm compared to IB and deals are negotiable. $100k at my firm could get you $3M in BP.
Lower commissions and more flexible leverage plus solid API offerings are all valid reasons. Thing is when I contacted a 'prop' firm previously, the commission quoted was higher than IB plus software/data fees, making me wonder why the heck anyone would consider using them
A lot of the prop firms I was looking at back then eventually disappeared and/or turned out to be scams. Nonko, anyone? In hindsight, sticking with IB was a great choice for that reason alone. Plus I never did come up with a strategy that could safely use more leverage than portfolio margin anyways (~6x)