This is a great question and to answer it we need to look at four different factors; geography, politics, trade and migration.

Geography
During the time of Imperial Russia (post sale of Alaska), the Siberian coastline ran over 2,500 miles from the Bering Sea to the Sea of Japan. Whilst fertile land that could support human settlement could be found in the southern regions of Priamursky and Magadan, the further North you went the more inhospitable the terrain became. Supporting anything more than small communities was extremely hard due to the waters there being incredibly rough (though full of fish) and the winters so brutal. If we take the most fertile and successful Russian settlement in the East - Vladivostok - and compare it to the United States of America, we find that Vladivostok is on a similar latitude to the Northern parts of California/Southern Oregon. If we were to compare the next largest Coastal town of Magan, you find it on a latitude similar to Anchorage. As such this huge expanse and degradation of climate led to the prospect of colonisation along the coast being both improbably and unprofitable. Which leads me to...

Trade
In contrast to the American journey West, the Russian migration East had been fuelled by a quest for furs, not land. The initial push had been almost 100% focused on gathering these furs, but once these had begun to run out a different approach was taken. In 1686 the treaty of Nerchinks was signed with the Chinese Empire demarcating the borders of the Russian and Chinese states. This in turn led to caravans setting out to buy Chinese goods and dozens of border towns, such as Aigun popping up. This trade was limited, however, by the sheer distance from Beijing to Moscow through only partially controlled and pacified territories. In the later part of the 19th century, Russian influence grew in the region as their natural population began to blossom in full. The Aigun treaty of 1858 ceded a large portion of the fertile Manchurian region (bordering at the Amur river) to Russia and this was colonised by Cossack and free Russian peasantry. This was seen as a valuable and important tract of land, which increased the Russian access too and control of the Chinese trade market dramatically.

Migration
As of 1861 the serfs of Russia were emancipated under Alexander II. This, however, was not a sudden and total freedom. Serfs were still required to work of debts they had to incur to pay for said freedom. Many were free on paper only, with the nobility owed outlandish sums which many simply would be unable to pay. Imperial Russia, much like modern Russia, used a passport system where migration could be controlled and had to be approved. This is turn limited migration throughout the Russian Empire. With regard to Siberia, where migration did occur it generally went in two directions; to the Siberian heartlands of Omsk and Novosibirsk and to the fertile far East of Priamursky. This migration was usually at the behest of state politics, which lastly leads me too...

Politics
Whilst in possession of an area three times the size of European Russia, for all of its history the population has been concentrated in the European part of Russia. As such, the focus of the state was largely left here and little hedence was paid to the Siberian coastline. This attitude did change, however, first during the 1904-5 Russo-Japanese War, and later in the Russian Civil War where foreign interventions highlighted the insecurities of the region.

Hey there,

Just to let you know, your question is fine, and we're letting it stand. However, you should be aware that questions framed as 'Why didn't X do Y' relatively often don't get an answer that meets our standards (in our experience as moderators). There are a few reasons for this. Firstly, it often can be difficult to prove the counterfactual: historians know much more about what happened than what might have happened. Secondly, 'why didn't X do Y' questions are sometimes phrased in an ahistorical way. It's worth remembering that people in the past couldn't see into the future, and they generally didn't have all the information we now have about their situations; things that look obvious now didn't necessarily look that way at the time.

If you end up not getting a response after a day or two, consider asking a new question focusing instead on why what happened did happen (rather than why what didn't happen didn't happen) - this kind of question is more likely to get a response in our experience. Hope this helps!