Why were so many baptists calvinist in the 1600's?

"In 1660 the General Baptists had already adopted A Brief Confession or Declaration of the Faith, in order to "set forth by many of us, who are falsely called Ana-Baptists to inform all men (in these days of scandal and reproach) of our innocent Belief and Practice; for which  we are not only resolved  to suffer Persecution, to the loss of our Goods, but also life it self, rather than do the same."  With this political and theological purpose in mind, why would the Assembly, or churches affiliated with it, feel a need to establish a new document?
  The answer tot his question is twofold.  First was the political reason.  Shortly after the presentation of the 1660 confession, under the leadership of Thomas Venner, the fifth Monarchists broke into rebellion and caused dissenters and especially Baptists to be looked upon with greater suspicion2 B.R.White suggests that "Fifth monarchy views were regarded as politically dangerous and that the authorities did not attempt to make any distinction between those who were relatively harmless Bible students and those who were potential or actual revolutionaries."3  This hazardous situation created a bond between the dissenting factions, and with the act of uniformity in 1662 adding Presbyterians to their ranks, an increased conglomerate of opposition led the Baptists to seek uniformity with other factions who were fighting not just against the Church of England but against the threat of Popery from King Charles II.5 In 1677 the Particular Baptist presented their 2nd London Baptist Confession.  Then in 1678 when "an ebullition of anti-Roman wrath swept through the nation:6 the General Baptist of the midlands followed what the Particular Baptist had done the year previous sand presented a creed.  The subtitle of this creed, as contrasted with the previous one, was simply, An Essay to unite and conform all true Protestants in the Fundamental Articles of the Christian Religion Against the errors and Heresies of the church of Rome/"
   Unity was the main purpose of the Orthodox Creed.  William H. Brackney in discussing the desire to present more mediating positions of though among General Baptists, calls it "The  capstone document of mediating confessions."7 Following the form of the Westminster Confession, and subsequently that of the Particular Baptist, was not the only way in which a sense of community with other dissenters was sought in this confession.  As Underwood says, "It's articles on Election, Reprobation, and Original Sin, and Perseverance were nothing like as Arminian in tone as John  Smyth or Thomas Helwys would have made them."8
Southwestern Journal of Theology "British Baptists" Transcribers preface pg. 127-129

We have to remember that creeds and confessions are scarcely if ever free from the tampering of political bias. We can even see this int he spread of the Baptist Faith an message 2000. Changes in wording are often ignored so long as major political factors are dealt with. In the case of BF&M2000 many quickly changed over either for the sake of the issue of inerrancy or more importantly for an official statement for traditional marriage. as many similar documents were drawn in hopes of legal protection.