The short answer is that Spain was much too busy. That, and the wealth and power of the Spanish Empire had been in a long decline since the glory days of the 16th century. They simply did not have the resources to devote to such an enormous undertaking.

Spain (and most of the rest of Europe) was still embroiled in the Thirty Years' War (1618-1648) when the English Civil War began in 1642. Spain and France would remain at war following the Peace of Westphalia in 1648, further preventing the Spanish government from turning much attention away from their wars on the continent. The Franco-Spanish war would continue until 1659.

In fact, Spain was very quick to recognize the Regicide English Republic after Charles I was beheaded in 1649. It may be that the Spanish were desperate for peace at any cost, even if that meant siding with a rebel Protestant Republic who had just executed a Catholic monarch. Spanish diplomatic recognition was likewise rapidly extended to Oliver Cromwell after he established himself as virtual dictator of the Protectorate in 1653.

There were even (apparently) serious overtures at an Anglo-Spanish alliance early in 1654, however this came to an abrupt end when Cromwell launched his secret "Western Design" against the Spanish Caribbean in 1655. This new round of Anglo-Spanish warfare pushed the precarious Spanish Empire further off-balance, and it is likely that by the 1670s it would never again be possible for Spain to threaten an invasion of the British Isles.

Battick, John. “Cromwell’s Diplomatic Blunder: The Relationship Between the Western Design of 1654-55 and the French Alliance of 1657”, Albion: A Quarterly Journal Concerned with British Studies 5, (Winter, 1973): 279-298.

I assume you mean the mid-17th century Civil War.

At that time, Spain was involved in the 30 Years War on the Continent, particularly trying to hang on to a least a part of the Netherlands, which was under attack from France. Additionally, thanks in part to failures like the Spanish Armada, the Spanish crown had very little money. Thus, despite the turmoil in England, Spain was in no position to act.

/u/darwinfish86 is on the ball here; Spain was too busy fighting its own battles, not just against other continental powers but also working to supress Catalonian revolts during the 1640s exacerbated by the strain of many years at war. The Irish Confederates (at war on-and-off with all parties in the English Civil War) lobbied the Spanish government to send aid but found themselves sidelined by Spain's primary interests and a Spanish unwillingness to offend Charles I in what was a moment of weakness of their own. There was a pretext here in that Spain had landed troops in Ireland in 1601 in support of Irish rebels and in an attempt to force open a front against Elizabeth I's England.

The Spanish did send representatives to treat with the Confederate Assembly but used these embassies to unsuccessfully recruit Irish soldiers for their continental wars rather than provide any substantial assistance to the Irish rebels. The most support they offered was to not impede thousands of Irish officers and soldiers in the service of Spanish armies returning to Ireland and forming the core of the Confederate armies.

Hiram Morgan argues that news from Ireland may actually have been seized upon by Catalan rebels as inspirational material; comparisons being drawn between Habsburg Catalonia and Stuart Ireland as subjects of a larger multiple monarchy suffering vaguely similar crises and a division of loyalties between king and country (the Confederates claimed loyalty to Charles but flirted heavily with the idea of becoming a republic on Dutch lines). The initial successes of Irish rebels in the early 1640s were seized upon by malcontents within Spain, and only further distracted Spain from taking advantage of England's own weakness.

'Foreign Intervention' in Ó Siochrú (2008) God's Executioner.

Hiram Morgan 'News from Ireland: Catalan, Portuguese and Castilian pamphlets on the Confederate War in Ireland' in Ó Siochrú and Ohlmeyer (2013) Ireland, 1641: Contexts and Reactions.