Micro-payments are like any other pay-wall -- they imply throwing away the vast majority of the traffic you get from search engines and links.
They've never been rolled out in a serious way, because advertisers are currently paying enough for the unfiltered masses to outweigh the revenue that could be generated by those readers that survive a pay-wall.
As for why those with pay-walls don't use the micro-transaction model -- they can't charge people for what they actually read without the per-story price being large enough to trigger that discrete psychological purchase decision that micro-payments' tiny size was envisioned to avoid.
E.g. An average person paying $5/mo for the WSJ might read 50 stories a month. Ask them for a dime a story and they'll read fewer, reducing your revenue. With subscription pricing they essentially make all that stuff the reader doesn't really want appear to be a feature that, for whatever reason, they do value enough to pay for.
They've never been rolled out in a serious way, because advertisers are currently paying enough for the unfiltered masses to outweigh the revenue that could be generated by those readers that survive a pay-wall.
As for why those with pay-walls don't use the micro-transaction model -- they can't charge people for what they actually read without the per-story price being large enough to trigger that discrete psychological purchase decision that micro-payments' tiny size was envisioned to avoid.
E.g. An average person paying $5/mo for the WSJ might read 50 stories a month. Ask them for a dime a story and they'll read fewer, reducing your revenue. With subscription pricing they essentially make all that stuff the reader doesn't really want appear to be a feature that, for whatever reason, they do value enough to pay for.