Shamelessly copying the Bioware wikipedia page (middle paragraph bold faced by me):
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Their first game, Shattered Steel, began its life as a proof-of-concept demo, similar to the MechWarrior games. This demo was submitted to ten publishers, seven of whom returned to the company with an offer. A publishing deal for Shattered Steel was eventually signed with Interplay Entertainment. Brent Oster and Trent Oster left BioWare at that time to form Pyrotek Studios, which continued developing Shattered Steel but broke up a year later, with Trent returning to BioWare to finish the game.[5] BioWare's first game was released the following year. Shattered Steel's release was described by IGN as a "modest success" with "decent sales". Two noteworthy points were the deformable terrain (player weapon damage caused craters in the environment) and zone damage (well-placed gunfire could shoot mounted weapons off enemies). A sequel to Shattered Steel was planned for 1998 but never realized.[4]
BioWare's founders and staff were keenly interested in both computerized and pen-and-paper variants of role-playing games. Their next development project, therefore, was determined to be a role-playing game. When Interplay financed "exploratory development", BioWare presented the publishers with a demo called Battleground: Infinity. Interplay suggested that the demonstrated gameplay engine would be well-suited to the Dungeons & Dragons licence which it had acquired from Strategic Simulations. Accordingly, Infinity was reworked in line with the Dungeons & Dragons ruleset.[4]
This resulted in Baldur's Gate, which witnessed a development cycle of three years. During this time, the three doctors continued to practice medicine. However, in the final years of the project, the demands of development prompted Muzyka and Zeschuck to leave medicine and move into full-time development. Augustine Yip decided to continue with his medical practice. Baldur's Gate sold more than two million copies after its release, nearly matching the sales of Diablo. Following the success of Baldur's Gate, the Infinity Engine was used for the games Planescape: Torment and the Icewind Dale series. The success of Baldur's Gate was followed by an expansion pack for the game: Tales of the Sword Coast.[6]
Edit: tl;dr; what was pitched as a typical (for the time) rts game got reworked into Baldurs Gate instead. Based on memory, may be wrong, but the thinking behind it being that the rts market was already very competitive at the time.