I agree that PKR hasn't been generous as to rake races... well, it's facing so many issues now that its inclination not to offer promos for grinders is understandable - it wants to operate on thick margins now. Because the network has True Value system in place that awards a skin less money for $1 of regs' rake than for $1 of recs' rake, it's rather risky for a skin to give much to regs, it might end up suffering losses on them.
PKR's big disadvantage is that it's a single brand on the network, whereas the Betsson group has as many as 6 brands and apparently one's accounts with separate brands are treated separately by the True Value system, i.e. a new account with a new skin is valued like an average player, not like an existing reg, and it's +EV to make a player use the signup bonuses of all or most of the skins owned by the company.
All that said,
the True Value system is going to be modernised soon, which might change the situation quite a bit.