I'm afraid the answer is rather simple. They were supposed to be a venture: the Plymouth Colony was funded by a loose group of merchants, assembled by an entrepreneur named Thomas Weston. While the Pilgrims ( or Brownists ) waited in Leiden, Weston was supposed to arrange for the transport. That he didn't get arranged until June of 1621: and the Brownists were told this was going to be their only chance, so they came from Leiden. Then fitting out the ships for the voyage ( one would be found unfit, so they had to make do with just the Mayflower), and augmenting the numbers of Brownists with "Strangers" deemed to be necessary ( like the soldier John Standish) took yet more time. We don't know a lot about Weston: he eventually ended up in Maryland, apparently having tried a number of ways to make money in the new English colonies. But none of his ventures were very successful, and perhaps the Brownists would have later said that sending them over in September was a good indication of his capability, as half of them died that first winter. Even more of an indication was to come a year later, when Weston sent them more people, no supplies, and wanted to know why the Mayflower had been sent back to England empty of profitable goods, in April 1622; a bare five months after they'd landed.