To achieve economies of scope, most motor carriers combine long‐term contracts with shippers and brokers with periodic spot assignments found on electronic marketplaces (EMs). While previous research has addressed how carriers adopt an EM, we know little about factors that influence carriers to adopt multiple EMs. Given the rise of the platform economy of the trucking industry, we chose to address this gap and generate mid‐range theory on adopting multiple EMs in a logistics context. To do this, we applied grounded theory and conducted 23 interviews with motor carriers and EM experts in North America and Europe until we reached theoretical saturation. Our findings reveal that many motor carriers adopt a portfolio of different EMs, and that their awareness of platforms, expected and realized benefits, attitude, and vigilance determine how they configure their EM portfolios. The implication for existing theory is that, while previous studies depicted EM adoption from a single‐system perspective, we found that it is actually a continuous selection process that follows a portfolio perspective. Our paper also has implications for practice in that it illuminates the rationales behind EM portfolio development and identifies actionable factors that can help managers configure stronger portfolios.